Relanding https://codereview.webrtc.org/1715883002/ in pieces.

- Clean up unused methods in voe::Channel following removal of VoEDtmf APIs.

BUG=webrtc:4690

Review URL: https://codereview.webrtc.org/1785643006

Cr-Commit-Position: refs/heads/master@{#11976}

webrtc/modules/audio_coding/test/TwoWayCommunication.cc

@@ -50,14 +50,6 @@ TwoWayCommunication::~TwoWayCommunication() {
   delete _channel_B2A;
   delete _channelRef_A2B;
   delete _channelRef_B2A;
-#ifdef WEBRTC_DTMF_DETECTION
-  if (_dtmfDetectorA != NULL) {
-    delete _dtmfDetectorA;
-  }
-  if (_dtmfDetectorB != NULL) {
-    delete _dtmfDetectorB;
-  }
-#endif
   _inFileA.Close();
   _inFileB.Close();
   _outFileA.Close();