Delete support for sending RTCP RPSI and SLI messages.

BUG=webrtc:7338

Review-Url: https://codereview.webrtc.org/2746413003
Cr-Commit-Position: refs/heads/master@{#17229}

webrtc/modules/video_coding/video_coding_impl.h

@@ -207,7 +207,6 @@ class VideoReceiver : public Module {
   int32_t Decode(const webrtc::VCMEncodedFrame& frame)
       EXCLUSIVE_LOCKS_REQUIRED(receive_crit_);
   int32_t RequestKeyFrame();
-  int32_t RequestSliceLossIndication(const uint64_t pictureID) const;
 
  private:
   rtc::ThreadChecker construction_thread_;