Allow mocking SendSideCongestionController for Call tests.

webrtc/call/rtp_transport_controller_send.cc

Index: webrtc/call/rtp_transport_controller_send.cc
diff --git a/webrtc/call/rtp_transport_controller_send.cc b/webrtc/call/rtp_transport_controller_send.cc
index a006f289f6e0de6dc2b3eb7fc9b0e8983a45f94d..1040218c03fc24a87debdf32bbcbdf853f37a052 100644
--- a/webrtc/call/rtp_transport_controller_send.cc
+++ b/webrtc/call/rtp_transport_controller_send.cc
@@ -18,10 +18,4 @@ RtpTransportControllerSend::RtpTransportControllerSend(
     : send_side_cc_(clock, nullptr /* observer */, event_log, &packet_router_) {
 }
 
-void RtpTransportControllerSend::RegisterNetworkObserver(
-    SendSideCongestionController::Observer* observer) {
-  // Must be called only once.
-  send_side_cc_.RegisterNetworkObserver(observer);
-}
-
 }  // namespace webrtc