Clean up decoders when stopping video receive stream.

This updates VideoReceiveStream2::Stop() to symmetrically tear down
state that's built up in VideoReceiveStream2::Start().

Bug: webrtc:11993, webrtc:14486
Change-Id: I41f4feea5584e5baaeed2143432136f8b9761321
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272537
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38244}

modules/video_coding/decoder_database.h

@@ -40,6 +40,7 @@ class VCMDecoderDataBase {
   void RegisterReceiveCodec(uint8_t payload_type,
                             const VideoDecoder::Settings& settings);
   bool DeregisterReceiveCodec(uint8_t payload_type);
+  void DeregisterReceiveCodecs();
 
   // Returns a decoder specified by frame.PayloadType. The decoded frame
   // callback of the decoder is set to `decoded_frame_callback`. If no such