Delete RtpReceiverImpl::CheckPayloadChanged.

Also delete related code in RtpReceiverAudio, RtpReceiverVideo and
RtpPayloadRegistry.

Only intended change in behavior is that packets with unknown payload
types are not discarded at this level of the stack. They are discarded
higher up, in Channel::ReceivePacket (audio) and
RtpVideoStreamReceiver::ReceivePacket (video).

Bug: webrtc:8995
Change-Id: I807997120bb40a95b0575c55db6e20a0cac651bf
Reviewed-on: https://webrtc-review.googlesource.com/92087
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24196}

modules/rtp_rtcp/source/rtp_receiver_impl.h

@@ -66,8 +66,6 @@ class RtpReceiverImpl : public RtpReceiver {
  private:
   void CheckSSRCChanged(const RTPHeader& rtp_header);
   void CheckCSRC(const WebRtcRTPHeader& rtp_header);
-  int32_t CheckPayloadChanged(const RTPHeader& rtp_header,
-                              PayloadUnion* payload);
 
   void UpdateSources(const absl::optional<uint8_t>& ssrc_audio_level);
   void RemoveOutdatedSources(int64_t now_ms);