Remove default implementation of PeerConnectionInterface::SetBitrate.

This was included to avoid breaking chromium, which now includes its own implementation (725cb26dab).

BUG=webrtc:7395

Review-Url: https://codereview.webrtc.org/2924243003
Cr-Commit-Position: refs/heads/master@{#19063}

webrtc/api/peerconnectioninterface.h

@@ -762,9 +762,7 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   //
   // Setting |current_bitrate_bps| will reset the current bitrate estimate
   // to the provided value.
-  virtual RTCError SetBitrate(const BitrateParameters& bitrate) {
-    return RTCError::OK();
-  }
+  virtual RTCError SetBitrate(const BitrateParameters& bitrate) = 0;
 
   // Returns the current SignalingState.
   virtual SignalingState signaling_state() = 0;