Remove VideoReceiveStream deregister of decoders.

Also doing some simplifications inside video_coding. No CHECKs added,
since they appear to have introduced breakages in downstream tests.

Overall reducing the number of potential ways a decoder could possibly
be set null. Removing deregistration of external decoders should also
give a quicker shutdown time since that may attempt to register
internal decoders.

BUG=chromium:563299
TBR=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1483423002 .

Cr-Commit-Position: refs/heads/master@{#10858}

webrtc/modules/video_coding/codec_database.h

@@ -54,8 +54,6 @@ class VCMCodecDataBase {
   // Returns the default settings for the codec with type |codec_type|.
   static void Codec(VideoCodecType codec_type, VideoCodec* settings);
 
-  void ResetSender();
-
   // Sets the sender side codec and initiates the desired codec given the
   // VideoCodec struct.
   // Returns true if the codec was successfully registered, false otherwise.
@@ -88,9 +86,6 @@ class VCMCodecDataBase {
 
   bool SetPeriodicKeyFrames(bool enable);
 
-  // Receiver Side
-  void ResetReceiver();
-
   // Deregisters an external decoder object specified by |payload_type|.
   bool DeregisterExternalDecoder(uint8_t payload_type);