Revert "Reland "Activate adaptive channel mixing when the echo canceller runs in stereo""

This reverts commit 5ad42502a816e90fcb8ff1d157267f676fe8ebc9.

Reason for revert: broke the chromium roll.

Original change's description:
> Reland "Activate adaptive channel mixing when the echo canceller runs in stereo"
>
> This reverts commit 8c9f8a0fc026f856961845e4cb835237aeb61304.
>
> Reason for revert: The issues in code that this CL relies on have now been addressed and the CL should now work as intended.
>
> Original change's description:
> > Revert "Activate adaptive channel mixing when the echo canceller runs in stereo"
> >
> > This reverts commit 82a3659e1164aeb4f3ace437a061684734786d50.
> >
> > Reason for revert: Seems to be breaking in chromium:
> > https://chromium-review.googlesource.com/c/chromium/src/+/7490942?tab=checks
> >
> > #
> > # Fatal error in: third_party/webrtc/modules/audio_processing/audio_buffer.cc, line 152
> > # last system error: 2
> > # Check failed: !use_adaptive_downmixing || buffer_internal_num_channels_ == 2
> > # 
> >
> > Original change's description:
> > > Activate adaptive channel mixing when the echo canceller runs in stereo
> > >
> > > This CL activates adaptive channel mixing when the echo canceller runs in stereo mode.
> > > The mixing is provided by the CaptureRemixer class.
> > > The adaptive mixing is activated behind a finch-based kill-switch that allows it to be
> > > activated.
> > >
> > > Bug: webrtc:42221468
> > > Change-Id: I5c4e79edb69752bbbb4c1cfe9271d1224fb695aa
> > > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/433640
> > > Commit-Queue: Per Åhgren <peah@webrtc.org>
> > > Reviewed-by: Tomas Lundqvist <tomasl@google.com>
> > > Cr-Commit-Position: refs/heads/main@{#46678}
> >
> > Bug: webrtc:42221468
> > Change-Id: Ifc3f489e0fe8b4da1b891fd0dd425280a8698fdf
> > Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442441
> > Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> > Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> > Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
> > Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> > Cr-Commit-Position: refs/heads/main@{#46685}
>
> Bug: webrtc:42221468
> Change-Id: I720df6d70de705a1027907d3cd2758d6b569cd75
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/444140
> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Jesus de Vicente Pena <devicentepena@webrtc.org>
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Commit-Queue: Per Åhgren <peah@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#46800}

Bug: webrtc:42221468
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: If52e241869547996452f62aff9baba943f459723
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/446340
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46809}
diff --git a/api/audio/audio_processing.h b/api/audio/audio_processing.h
index 878059b..d257929 100644
--- a/api/audio/audio_processing.h
+++ b/api/audio/audio_processing.h
@@ -156,7 +156,7 @@
       // needed).
       DownmixMethod capture_downmix_method = DownmixMethod::kAverageChannels;
       DownmixMethod capture_downmix_method_stereo_aec =
-          DownmixMethod::kAdaptive;
+          DownmixMethod::kAverageChannels;
     } pipeline;
 
     // Enabled the pre-amplifier. It amplifies the capture signal
diff --git a/experiments/field_trials.py b/experiments/field_trials.py
index 537fb67..9faa63a 100755
--- a/experiments/field_trials.py
+++ b/experiments/field_trials.py
@@ -53,9 +53,6 @@
     FieldTrial('WebRTC-Agc2SpeechLevelEstimatorExperimental',
                457791164,
                date(2026, 11, 5)),
-    FieldTrial('WebRTC-ApmAdaptiveCaptureMixingForStereoKillSwitch',
-               42221468,
-               date(2026, 8, 1)),
     FieldTrial('WebRTC-ApmEnforce48kHzProcessingRate',
                454695115,
                date(2026, 6, 1)),
diff --git a/modules/audio_processing/audio_processing_impl.cc b/modules/audio_processing/audio_processing_impl.cc
index 6857eb3..e86cc3c 100644
--- a/modules/audio_processing/audio_processing_impl.cc
+++ b/modules/audio_processing/audio_processing_impl.cc
@@ -474,9 +474,7 @@
               "WebRTC-ApmExperimentalMultiChannelCaptureKillSwitch"),
           EnforceSplitBandHpf(env.field_trials()),
           MinimizeProcessingForUnusedOutput(env.field_trials()),
-          env.field_trials().IsEnabled("WebRTC-ApmEnforce48kHzProcessingRate"),
-          env.field_trials().IsEnabled(
-              "WebRTC-ApmAdaptiveCaptureMixingForStereoKillSwitch")),
+          env.field_trials().IsEnabled("WebRTC-ApmEnforce48kHzProcessingRate")),
       capture_(),
       capture_nonlocked_(),
       applied_input_volume_stats_reporter_(
@@ -566,20 +564,12 @@
     render_.render_converter.reset(nullptr);
   }
 
-  const bool use_echo_controller =
-      NeedEchoController(config_, !!echo_control_factory_);
-  const bool multi_channel_capture = config_.pipeline.multi_channel_capture &&
-                                     constants_.multi_channel_capture_support;
   // Enforce adaptive downmixing when the echo canceller is active and
   // multi-channel processing is used.
   AudioProcessing::Config::Pipeline::DownmixMethod downmixing_method =
       config_.pipeline.capture_downmix_method;
   AudioProcessing::Config::Pipeline::DownmixMethod downmixing_method_stereo =
       config_.pipeline.capture_downmix_method_stereo_aec;
-  if (constants_.fallback_to_generic_downmixing_with_stereo_aec ||
-      (!(use_echo_controller && multi_channel_capture))) {
-    downmixing_method_stereo = downmixing_method;
-  }
 
   capture_.capture_audio.reset(new AudioBuffer(
       formats_.api_format.input_stream().sample_rate_hz(),
diff --git a/modules/audio_processing/audio_processing_impl.h b/modules/audio_processing/audio_processing_impl.h
index 6db177b..c058961 100644
--- a/modules/audio_processing/audio_processing_impl.h
+++ b/modules/audio_processing/audio_processing_impl.h
@@ -424,23 +424,19 @@
                  bool multi_channel_capture_support,
                  bool enforce_split_band_hpf,
                  bool minimize_processing_for_unused_output,
-                 bool enforce_48_khz_max_internal_processing_rate,
-                 bool fallback_to_generic_downmixing_with_stereo_aec)
+                 bool enforce_48_khz_max_internal_processing_rate)
         : multi_channel_render_support(multi_channel_render_support),
           multi_channel_capture_support(multi_channel_capture_support),
           enforce_split_band_hpf(enforce_split_band_hpf),
           minimize_processing_for_unused_output(
               minimize_processing_for_unused_output),
           enforce_48_khz_max_internal_processing_rate(
-              enforce_48_khz_max_internal_processing_rate),
-          fallback_to_generic_downmixing_with_stereo_aec(
-              fallback_to_generic_downmixing_with_stereo_aec) {}
+              enforce_48_khz_max_internal_processing_rate) {}
     bool multi_channel_render_support;
     bool multi_channel_capture_support;
     bool enforce_split_band_hpf;
     bool minimize_processing_for_unused_output;
     bool enforce_48_khz_max_internal_processing_rate;
-    bool fallback_to_generic_downmixing_with_stereo_aec;
   } constants_;
 
   struct ApmCaptureState {