Remove voe::OutputMixer and AudioConferenceMixer.

voice_engine/channel_proxy.cc

Index: voice_engine/channel_proxy.cc
diff --git a/voice_engine/channel_proxy.cc b/voice_engine/channel_proxy.cc
index f6013c2540de1e023c4281606b2e77380442d015..15db60d9b9bb8603bfb6fd749783718c4d0a63c6 100644
--- a/voice_engine/channel_proxy.cc
+++ b/voice_engine/channel_proxy.cc
@@ -257,9 +257,9 @@ AudioMixer::Source::AudioFrameInfo ChannelProxy::GetAudioFrameWithInfo(
   return channel()->GetAudioFrameWithInfo(sample_rate_hz, audio_frame);
 }
 
-int ChannelProxy::NeededFrequency() const {
+int ChannelProxy::PreferredSampleRate() const {
   RTC_DCHECK_RUNS_SERIALIZED(&audio_thread_race_checker_);
-  return static_cast<int>(channel()->NeededFrequency(-1));
+  return channel()->PreferredSampleRate();
 }
 
 void ChannelProxy::SetTransportOverhead(int transport_overhead_per_packet) {