Delete VideoReceiver2::TriggerDecoderShutdown.

This method used to be wired down to VCMReceiver and to
VCMJitterBuffer::Stop, but has become a nop. Also delete some
obsoleted comments.

Bug: webrtc:7408
Change-Id: I4c1e67272b1ffda786cc0ff358fa38e594aff304
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152620
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29167}

modules/video_coding/video_receiver2.h

@@ -40,8 +40,6 @@ class VideoReceiver2 {
 
   int32_t Decode(const webrtc::VCMEncodedFrame* frame);
 
-  void TriggerDecoderShutdown();
-
   // Notification methods that are used to check our internal state and validate
   // threading assumptions. These are called by VideoReceiveStream.
   // See |IsDecoderThreadRunning()| for more details.