Remove unused DBusThreadManagerObserver

chromeos/dbus/dbus_thread_manager_observer.h

-// Copyright (c) 2012 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 CHROMEOS_DBUS_DBUS_THREAD_MANAGER_OBSERVER_H_
-#define CHROMEOS_DBUS_DBUS_THREAD_MANAGER_OBSERVER_H_
-
-namespace chromeos {
-
-class DBusThreadManager;
-
-// Interface for classes that are interested in observing changes to the
-// DBusThreadManager.
-class DBusThreadManagerObserver {
- public:
-  // Called when |manager| is about to be destroyed.  |manager| is still usable
-  // at this point.
-  virtual void OnDBusThreadManagerDestroying(DBusThreadManager* manager) = 0;
-
- protected:
-  virtual ~DBusThreadManagerObserver() {}
-};
-
-}  // namespace chromeos
-
-#endif  // CHROMEOS_DBUS_DBUS_THREAD_MANAGER_OBSERVER_H_