Delete unsupported method VideoCodingModule::RegisterDecoderTimingCallback.

The implementation behind this method has been a noop for a long time.

BUG=none

Review-Url: https://codereview.webrtc.org/2757843002
Cr-Commit-Position: refs/heads/master@{#17286}

webrtc/modules/video_coding/video_coding_impl.cc

@@ -200,11 +200,6 @@ class VideoCodingModuleImpl : public VideoCodingModule {
     return receiver_.RegisterReceiveStatisticsCallback(receiveStats);
   }
 
-  int32_t RegisterDecoderTimingCallback(
-      VCMDecoderTimingCallback* decoderTiming) override {
-    return receiver_.RegisterDecoderTimingCallback(decoderTiming);
-  }
-
   int32_t RegisterFrameTypeCallback(
       VCMFrameTypeCallback* frameTypeCallback) override {
     return receiver_.RegisterFrameTypeCallback(frameTypeCallback);