Remove WmLookup.

ash/mus/bridge/wm_lookup_mus.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 ASH_MUS_BRIDGE_WM_LOOKUP_MUS_H_
-#define ASH_MUS_BRIDGE_WM_LOOKUP_MUS_H_
-
-#include <stdint.h>
-
-#include "ash/common/wm_lookup.h"
-#include "base/macros.h"
-
-namespace ash {
-namespace mus {
-
-// WmLookup implementation for mus.
-class WmLookupMus : public WmLookup {
- public:
-  WmLookupMus();
-  ~WmLookupMus() override;
-
-  // WmLookup:
-  RootWindowController* GetRootWindowControllerWithDisplayId(
-      int64_t id) override;
-  WmWindow* GetWindowForWidget(views::Widget* widget) override;
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(WmLookupMus);
-};
-
-}  // namespace mus
-}  // namespace ash
-
-#endif  // ASH_MUS_BRIDGE_WM_LOOKUP_MUS_H_