Delete class NullRtpData and function NullObjectRtpData.

BUG=webrtc:5565

Review-Url: https://codereview.webrtc.org/2885823002
Cr-Commit-Position: refs/heads/master@{#18366}

webrtc/modules/rtp_rtcp/source/rtcp_sender_unittest.cc

@@ -197,7 +197,7 @@ class RtcpPacketTypeCounterObserverImpl : public RtcpPacketTypeCounterObserver {
 };
 
 class TestTransport : public Transport,
-                      public NullRtpData {
+                      public RtpData {
  public:
   TestTransport() {}