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

ash/common/system/tray/tray_popup_label_button_border.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_BORDER_H_
-#define ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LABEL_BUTTON_BORDER_H_
-
-#include "base/macros.h"
-#include "ui/views/controls/button/label_button_border.h"
-
-namespace ash {
-
-// A border for label buttons that paints a vertical separator in normal state
-// and a custom hover effect in hovered or pressed state.
-class TrayPopupLabelButtonBorder : public views::LabelButtonAssetBorder {
- public:
-  TrayPopupLabelButtonBorder();
-  ~TrayPopupLabelButtonBorder() override;
-
-  // views::LabelButtonAssetBorder:
-  void Paint(const views::View& view, gfx::Canvas* canvas) override;
-
- private:
-  DISALLOW_COPY_AND_ASSIGN(TrayPopupLabelButtonBorder);
-};
-
-}  // namespace ash
-
-#endif  // ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LABEL_BUTTON_BORDER_H_