Fix stats for encoder target bitrate when target rate is zero.

When the target bitrate is zero, currently  VideoSendStream.Stats.target_media_bitrate_bps show the last set rate before the target was set to zero.

BUG=webrtc::5687 b/29574845

Review-Url: https://codereview.webrtc.org/2122743003
Cr-Commit-Position: refs/heads/master@{#13386}

webrtc/modules/video_coding/video_coding_impl.h

@@ -58,7 +58,6 @@ class VideoSender : public Module {
 
   VideoSender(Clock* clock,
               EncodedImageCallback* post_encode_callback,
-              VideoEncoderRateObserver* encoder_rate_observer,
               VCMSendStatisticsCallback* send_stats_callback);
 
   ~VideoSender();