ACM: Remove ACMVQMonCallback object

It was never used, and the underlying functionality was removed long
ago.

BUG=3520
R=kwiberg@webrtc.org

Review URL: https://codereview.webrtc.org/1365193003 .

Cr-Commit-Position: refs/heads/master@{#10083}
diff --git a/webrtc/modules/audio_coding/main/interface/audio_coding_module.h b/webrtc/modules/audio_coding/main/interface/audio_coding_module.h
index a585605..5194415 100644
--- a/webrtc/modules/audio_coding/main/interface/audio_coding_module.h
+++ b/webrtc/modules/audio_coding/main/interface/audio_coding_module.h
@@ -54,19 +54,6 @@
   virtual int32_t InFrameType(FrameType frame_type) = 0;
 };
 
-// Callback class used for reporting receiver statistics
-class ACMVQMonCallback {
- public:
-  virtual ~ACMVQMonCallback() {}
-
-  virtual int32_t NetEqStatistics(
-      const int32_t id,  // current ACM id
-      const uint16_t MIUsValid,  // valid voice duration in ms
-      const uint16_t MIUsReplaced,  // concealed voice duration in ms
-      const uint8_t eventFlags,  // concealed voice flags
-      const uint16_t delayMS) = 0;  // average delay in ms
-};
-
 class AudioCodingModule {
  protected:
   AudioCodingModule() {}