Remove deprecated config option.

Downstream usage is gone.

Bug: webrtc:42221435
Change-Id: Ic20c193adea24c5bfd49ca649707355c65dbd3d4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/489200
Auto-Submit: Erik Språng <sprang@webrtc.org>
Reviewed-by: Zoé Lepaul <zlep@webrtc.org>
Commit-Queue: Zoé Lepaul <zlep@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48190}

modules/pacing/pacing_controller.h

diff --git a/modules/pacing/pacing_controller.h b/modules/pacing/pacing_controller.h
index b918d61..a09a1ea 100644
--- a/modules/pacing/pacing_controller.h
+++ b/modules/pacing/pacing_controller.h
@@ -102,9 +102,6 @@
     // SetPacingRates() so that this limit will be upheld if
     // `drain_large_queues` is set.
     TimeDelta queue_time_limit = kMaxExpectedQueueLength;
-    // TODO(bugs.webrtc.org/42221435): Remove when downstream usage is gone.
-    [[deprecated("Keyframe flushing is now always enabled.")]]
-    bool keyframe_flushing = false;
     // Audio retransmission is prioritized before video retransmission packets.
     bool prioritize_audio_retransmission = false;
     // Configure separate timeouts per priority. After a timeout, a packet of