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/codecs/vp9/vp9_impl.h

@@ -149,8 +149,6 @@ class VP9DecoderImpl : public VP9Decoder {
 
   int Release() override;
 
-  int Reset() override;
-
   const char* ImplementationName() const override;
 
  private: