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/generic_decoder.cc

@@ -171,10 +171,6 @@ int32_t VCMGenericDecoder::Release() {
   return _decoder->Release();
 }
 
-int32_t VCMGenericDecoder::Reset() {
-  return _decoder->Reset();
-}
-
 int32_t VCMGenericDecoder::RegisterDecodeCompleteCallback(
     VCMDecodedFrameCallback* callback) {
   _callback = callback;