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.h

@@ -83,11 +83,6 @@ class VCMGenericDecoder {
   */
   int32_t Release();
 
-  /**
-  * Reset the decoder state, prepare for a new call
-  */
-  int32_t Reset();
-
   /**
   * Set decode callback. Deregistering while decoding is illegal.
   */