Removed MediaStreamTrackInterface::set_state

The track state should be implicitly set by the underlying source.
This removes the public method and cleans up how AudioRtpReceiver is created. Further more it cleans up how the RtpReceivers are destroyed.

Note that this cl depend on https://codereview.webrtc.org/1790633002.
BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1816143002

Cr-Commit-Position: refs/heads/master@{#12115}

webrtc/api/mediastreaminterface.h

@@ -92,8 +92,6 @@ class MediaStreamTrackInterface : public rtc::RefCountInterface,
   virtual bool enabled() const = 0;
   virtual TrackState state() const = 0;
   virtual bool set_enabled(bool enable) = 0;
-  // These methods should be called by implementation only.
-  virtual bool set_state(TrackState new_state) = 0;
 
  protected:
   virtual ~MediaStreamTrackInterface() {}