Video receiver: reduce data race

- do not destroy demux context on receiver thread

Gitlab: #732

Change-Id: I34e9922305073a606467d178f13606eb1d721d2f

src/media/video/video_receive_thread.cpp

@@ -154,7 +154,6 @@ VideoReceiveThread::cleanup()
     sink_->stop();
 
     videoDecoder_.reset();
-    demuxContext_.reset();
 }
 
 // This callback is used by libav internally to break out of blocking calls