Revert of Adding "first packet received" notification to PeerConnectionObserver. (patchset #5 id:80001 of https://codereview.webrtc.org/1581693006/ )

Reason for revert:
onFirstMediaPacketReceived() breaks bot.

Original issue's description:
> Adding "first packet received" notification to PeerConnectionObserver.
>
> R=glaznev@webrtc.org, pthatcher@webrtc.org, tkchin@webrtc.org
>
> Committed: https://crrev.com/42265a8cc3b3f3db4aa2c29005aed2fb4393adef
> Cr-Commit-Position: refs/heads/master@{#11401}
>
> Committed: https://crrev.com/08a6eab75e13613183509d91d3892c1db57f6fc5
> Cr-Commit-Position: refs/heads/master@{#11404}

TBR=pthatcher@webrtc.org,tkchin@webrtc.org,glaznev@webrtc.org,deadbeef@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.webrtc.org/1647483004

Cr-Commit-Position: refs/heads/master@{#11415}

talk/app/webrtc/peerconnectioninterface.h

@@ -501,9 +501,6 @@ class PeerConnectionObserver {
   // Called when the ICE connection receiving status changes.
   virtual void OnIceConnectionReceivingChange(bool receiving) {}
 
-  // Called when the first RTP packet is received.
-  virtual void OnFirstMediaPacketReceived() {}
-
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.
   ~PeerConnectionObserver() {}