Remove TrayPopupLabelButton (a pre-MD class) and its assets.

ash/common/system/tray/tray_popup_label_button.h

-// Copyright 2013 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_COMMON_SYSTEM_TRAY_TRAY_POPUP_LABEL_BUTTON_H_
-#define ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LABEL_BUTTON_H_
-
-#include "base/macros.h"
-#include "base/strings/string16.h"
-#include "ui/views/controls/button/label_button.h"
-
-namespace ash {
-
-// A label button with custom alignment, border and focus border.
-// TODO(estade): deprecated for MD. Use CreateTrayPopupButton instead.
-// See crbug.com/614453
-class TrayPopupLabelButton : public views::LabelButton {
- public:
-  TrayPopupLabelButton(views::ButtonListener* listener,
-                       const base::string16& text);
-  ~TrayPopupLabelButton() override;
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(TrayPopupLabelButton);
-};
-
-}  // namespace ash
-
-#endif  // ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LABEL_BUTTON_H_