Remove framemarking RTP extension.

BUG=webrtc:11637

Change-Id: I47f8e22473429c9762956444e27cfbafb201b208
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176442
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31522}

modules/video_coding/frame_object.cc

@@ -129,9 +129,5 @@ const RTPVideoHeader& RtpFrameObject::GetRtpVideoHeader() const {
   return rtp_video_header_;
 }
 
-const FrameMarking& RtpFrameObject::GetFrameMarking() const {
-  return rtp_video_header_.frame_marking;
-}
-
 }  // namespace video_coding
 }  // namespace webrtc