[chromecast] Deletes dead code to track MediaPipelineBackend.

chromecast/media/cma/pipeline/media_pipeline_client.h

Index: chromecast/media/cma/pipeline/media_pipeline_client.h
diff --git a/chromecast/media/cma/pipeline/media_pipeline_client.h b/chromecast/media/cma/pipeline/media_pipeline_client.h
index 66a949dfec0f06902a473d8b1651886b47721e5c..1d980be28b91b4d77cd0f02dc5efb61b4702d996 100644
--- a/chromecast/media/cma/pipeline/media_pipeline_client.h
+++ b/chromecast/media/cma/pipeline/media_pipeline_client.h
@@ -30,11 +30,6 @@ struct MediaPipelineClient {
 
   // Callback used to report the buffering status.
   ::media::BufferingStateCB buffering_state_cb;
-
-  // Callback used to report the creation and destruction of
-  // MediaPipelineBackend
-  base::Closure pipeline_backend_created_cb;
-  base::Closure pipeline_backend_destroyed_cb;
 };
 
 }  // namespace media