Remove VCMRenderBufferSizeCallback.

Unused/dead code.

BUG=
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1923713002 .

Cr-Commit-Position: refs/heads/master@{#12595}

webrtc/modules/video_coding/receiver.h

@@ -90,11 +90,6 @@ class VCMReceiver {
   void SetDecodeErrorMode(VCMDecodeErrorMode decode_error_mode);
   VCMDecodeErrorMode DecodeErrorMode() const;
 
-  // Returns size in time (milliseconds) of complete continuous frames in the
-  // jitter buffer. The render time is estimated based on the render delay at
-  // the time this function is called.
-  int RenderBufferSizeMs();
-
   void RegisterStatsCallback(VCMReceiveStatisticsCallback* callback);
 
   void TriggerDecoderShutdown();