Revert of Adding "first packet received" notification to PeerConnectionObserver. (patchset #4 id:60001 of https://codereview.webrtc.org/1581693006/ )

Reason for revert:
Seems that the end-to-end unit tests are now flaky: https://build.chromium.org/p/client.webrtc/builders/Win64%20Debug/builds/6283

Will reland after fixing the test flakiness.

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}

TBR=pthatcher@webrtc.org,tkchin@webrtc.org,glaznev@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/1640173004

Cr-Commit-Position: refs/heads/master@{#11402}

webrtc/examples/androidapp/src/org/appspot/apprtc/PeerConnectionClient.java

@@ -1000,10 +1000,6 @@ public class PeerConnectionClient {
       // No need to do anything; AppRTC follows a pre-agreed-upon
       // signaling/negotiation protocol.
     }
-
-    @Override
-    public void onFirstMediaPacketReceived() {
-    }
   }
 
   // Implementation detail: handle offer creation/signaling and answer setting,