Deprecate VideoDecoder::Reset() and remove calls.

Removes calls to decoder reset and instead drops delta frames and
requests keyframes until one arrives.

BUG=webrtc:5475
R=stefan@webrtc.org
TBR=mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1647163002 .

Cr-Commit-Position: refs/heads/master@{#11460}

webrtc/modules/video_coding/video_coding_impl.cc

@@ -219,8 +219,6 @@ class VideoCodingModuleImpl : public VideoCodingModule {
     return receiver_.Decode(maxWaitTimeMs);
   }
 
-  int32_t ResetDecoder() override { return receiver_.ResetDecoder(); }
-
   int32_t ReceiveCodec(VideoCodec* currentReceiveCodec) const override {
     return receiver_.ReceiveCodec(currentReceiveCodec);
   }