Remove AudioProcessing::gain_control() getter

This change also resolves a bug in audioproc_f:
The implicit ApplyConfig calls to enable gain control settings in
aec_dump_simulator.cc:377-406 [1] are overwritten by the ApplyConfig
call on line 500 using a config from line 292.

Compared to a ToT build including a fix for that bug, these changes
are bitexact on a large number of aecdumps.

[1] https://cs.chromium.org/chromium/src/third_party/webrtc/modules/audio_processing/test/aec_dump_based_simulator.cc?l=377&rcl=8bbf9e2c6e40feb8efcbf276b43945a14d651e9b

Bug: webrtc:9878
Change-Id: Id427d34e838c999d996d58193977ac2a9198edd6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156463
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29481}

modules/audio_processing/test/debug_dump_test.cc

diff --git a/modules/audio_processing/test/debug_dump_test.cc b/modules/audio_processing/test/debug_dump_test.cc
index ff08e5d..b769a66 100644
--- a/modules/audio_processing/test/debug_dump_test.cc
+++ b/modules/audio_processing/test/debug_dump_test.cc
@@ -529,8 +529,9 @@
   generator.StartRecording();
   generator.Process(100);
 
-  GainControl* agc = generator.apm()->gain_control();
-  EXPECT_EQ(AudioProcessing::kNoError, agc->Enable(!agc->is_enabled()));
+  AudioProcessing::Config apm_config = generator.apm()->GetConfig();
+  apm_config.gain_controller1.enabled = !apm_config.gain_controller1.enabled;
+  generator.apm()->ApplyConfig(apm_config);
 
   generator.Process(100);
   generator.StopRecording();