views examples: remove obsolete CTA button example

ui/views/examples/button_example.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 UI_VIEWS_EXAMPLES_BUTTON_EXAMPLE_H_
 #define UI_VIEWS_EXAMPLES_BUTTON_EXAMPLE_H_
 
 #include "base/macros.h"
 #include "ui/views/controls/button/button.h"
 #include "ui/views/examples/example_base.h"
@@ skipping 21 matching lines @@
 
   // ButtonListener:
   void ButtonPressed(Button* sender, const ui::Event& event) override;
 
   // Example buttons.
   LabelButton* label_button_ = nullptr;
   LabelButton* styled_button_ = nullptr;
   LabelButton* disabled_button_ = nullptr;
   MdTextButton* md_button_ = nullptr;
   MdTextButton* md_default_button_ = nullptr;
-  MdTextButton* md_prominent_button_ = nullptr;
   ImageButton* image_button_ = nullptr;
 
   const gfx::ImageSkia* icon_ = nullptr;
 
   // The number of times the buttons are pressed.
   int count_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(ButtonExample);
 };
 
 }  // namespace examples
 }  // namespace views
 
 #endif  // UI_VIEWS_EXAMPLES_BUTTON_EXAMPLE_H_