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/video_coding_impl.cc

@@ -212,11 +212,6 @@ class VideoCodingModuleImpl : public VideoCodingModule {
     return receiver_.RegisterPacketRequestCallback(callback);
   }
 
-  int RegisterRenderBufferSizeCallback(
-      VCMRenderBufferSizeCallback* callback) override {
-    return receiver_.RegisterRenderBufferSizeCallback(callback);
-  }
-
   int32_t Decode(uint16_t maxWaitTimeMs) override {
     return receiver_.Decode(maxWaitTimeMs);
   }