Remove an unused MessageView#CreateOrUpdateCloseButtonView

This method is declared in the header but not defined anywhere.

Bug: none
Test: build passes

Change-Id: I6db055a785a549d1b1f24b5cb5b22400937ca241
Reviewed-on: https://chromium-review.googlesource.com/c/1295749Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602301}

ui/message_center/views/message_view.h

@@ -152,11 +152,6 @@ class MESSAGE_CENTER_EXPORT MessageView : public views::InkDropHostView,
   std::string notification_id() const { return notification_id_; }
 
  protected:
-  // Creates and add close button to view hierarchy when necessary. Derived
-  // classes should call this after its view hierarchy is populated to ensure
-  // it is on top of other views.
-  void CreateOrUpdateCloseButtonView(const Notification& notification);
-
   virtual void UpdateControlButtonsVisibility() = 0;
 
   // Changes the background color and schedules a paint.