[chromecast] Remove unused file.

MediaPipelineBackendFactory is not used.
CmaBackendFactory is used instead.

Bug: 149422033
Test: none
Change-Id: I920120c59b325f9a72848ddf46394a7f25f8c52b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163648Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Qingwen Pi <qwp@google.com>
Cr-Commit-Position: refs/heads/master@{#762096}

chromecast/media/api/media_pipeline_backend_factory.h

-// Copyright 2016 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.
-
-#ifndef CHROMECAST_MEDIA_API_MEDIA_PIPELINE_BACKEND_FACTORY_H_
-#define CHROMECAST_MEDIA_API_MEDIA_PIPELINE_BACKEND_FACTORY_H_
-
-#include <memory>
-
-#include "chromecast/public/media/media_pipeline_device_params.h"
-
-namespace chromecast {
-namespace media {
-
-class CmaBackend;
-
-// This class creates media backends.
-class MediaPipelineBackendFactory {
- public:
-  virtual ~MediaPipelineBackendFactory();
-
-  // Creates a CMA backend. Must be called on the same thread as
-  // |media_task_runner_|.
-  virtual std::unique_ptr<CmaBackend> CreateCmaBackend(
-      const MediaPipelineDeviceParams& params) = 0;
-}  // namespace media
-}  // namespace chromecast
-
-#endif  // CHROMECAST_MEDIA_API_MEDIA_PIPELINE_BACKEND_FACTORY_H_