Simplify some mojo_shell code.

mojo/shell/app_child_process.h

-// Copyright 2014 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 SHELL_APP_CHILD_PROCESS_H_
-#define SHELL_APP_CHILD_PROCESS_H_
-
-#include "base/macros.h"
-#include "mojo/shell/child_process.h"
-
-namespace mojo {
-namespace shell {
-
-// An implementation of |ChildProcess| for an app child process, which runs a
-// single app (loaded from the file system) on its main thread.
-class AppChildProcess : public ChildProcess {
- public:
-  AppChildProcess();
-  ~AppChildProcess() override;
-
-  void Main() override;
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(AppChildProcess);
-};
-
-}  // namespace shell
-}  // namespace mojo
-
-#endif  // SHELL_APP_CHILD_PROCESS_H_