[chromecast] Delete MediaMessageLoop.

chromecast/media/cma/backend/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("backend") {
   sources = [
     "audio_decoder_default.cc",
     "audio_decoder_default.h",
     "audio_decoder_wrapper.cc",
     "audio_decoder_wrapper.h",
     "media_pipeline_backend_default.cc",
     "media_pipeline_backend_default.h",
     "media_pipeline_backend_manager.cc",
     "media_pipeline_backend_manager.h",
     "media_pipeline_backend_wrapper.cc",
     "media_pipeline_backend_wrapper.h",
     "video_decoder_default.cc",
     "video_decoder_default.h",
   ]
 
   public_deps = [
     "//chromecast/public",
     "//chromecast/public/media",
   ]
 
   deps = [
     "//base",
-    "//chromecast/media/base:message_loop",
   ]
 }