Cleanup of unused RTP structs and packetizer for stereo codec

This CL is a followup to https://webrtc-review.googlesource.com/c/src/+/38481.
With the new approach we can just use the generic RTP packetizer to pass frames
over the wire as the specific info is contained within the bitstream. This makes
the new codec more modular and reduces its footprint.

Bug: webrtc:7671
Change-Id: Ib07f72a9d338e3cbfdbf39cba9891a959b5f7552
Reviewed-on: https://webrtc-review.googlesource.com/43220
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21753}
diff --git a/modules/video_coding/frame_object.cc b/modules/video_coding/frame_object.cc
index 6eb28de..6a31cfd 100644
--- a/modules/video_coding/frame_object.cc
+++ b/modules/video_coding/frame_object.cc
@@ -43,14 +43,9 @@
   frame_type_ = first_packet->frameType;
   codec_type_ = first_packet->codec;
 
-  // Stereo codec appends CopyCodecSpecific to last packet to avoid copy.
-  VCMPacket* packet_with_codec_specific =
-      codec_type_ == kVideoCodecStereo ? packet_buffer_->GetPacket(last_seq_num)
-                                       : first_packet;
-
   // TODO(philipel): Remove when encoded image is replaced by FrameObject.
   // VCMEncodedFrame members
-  CopyCodecSpecific(&packet_with_codec_specific->video_header);
+  CopyCodecSpecific(&first_packet->video_header);
   _completeFrame = true;
   _payloadType = first_packet->payloadType;
   _timeStamp = first_packet->timestamp;