Delete StreamStatistician::IsRetransmitOfOldPacket

Return value always passed as the |retransmitted| argument to
ReceiveStatistics::IncomingPacket. The implementation of this method,
StreamStatisticianImpl::IncomingPacket, can call its own
IsRetransmitOfOldPacket, which is demoted to a private method.

Bug: webrtc:7135
Change-Id: I904db676738689c7a1db4caa588f70e64e3c357d
Reviewed-on: https://webrtc-review.googlesource.com/95649
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24494}

video/video_receive_stream.cc

@@ -145,6 +145,9 @@ VideoReceiveStream::VideoReceiveStream(
         config_.rtp.remote_ssrc, rtp_receive_statistics_.get());
     rtx_receiver_ = receiver_controller->CreateReceiver(
         config_.rtp.rtx_ssrc, rtx_receive_stream_.get());
+  } else {
+    rtp_receive_statistics_->EnableRetransmitDetection(config.rtp.remote_ssrc,
+                                                       true);
   }
 }