Set StreamDataCountersCallback on construction of RTP modules

This CL sets the RTP stats callback on construction, by adding a field
next to the other observers in RtpRtcp::Configuration.
We can then remove the RegisterCallback() methods and the unused
GetCallback() method.

Bug: webrtc:11036
Change-Id: I4eb86ea63b4b2ebeff60b311ddf3bed06b279ce4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157169
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29504}

modules/rtp_rtcp/source/rtp_rtcp_impl.cc

@@ -741,16 +741,6 @@ int64_t ModuleRtpRtcpImpl::rtt_ms() const {
   return rtt_ms_;
 }
 
-void ModuleRtpRtcpImpl::RegisterSendChannelRtpStatisticsCallback(
-    StreamDataCountersCallback* callback) {
-  rtp_sender_->RegisterRtpStatisticsCallback(callback);
-}
-
-StreamDataCountersCallback*
-ModuleRtpRtcpImpl::GetSendChannelRtpStatisticsCallback() const {
-  return rtp_sender_->GetRtpStatisticsCallback();
-}
-
 void ModuleRtpRtcpImpl::SetVideoBitrateAllocation(
     const VideoBitrateAllocation& bitrate) {
   rtcp_sender_.SetVideoBitrateAllocation(bitrate);