Remove balloon notification code. The last user was the Linux GTK port but that's deleted now.

ui/message_center/dummy_message_center.cc

 // Copyright (c) 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.
 
 #include "ui/message_center/message_center.h"
 #include "ui/message_center/message_center_switches.h"
 #include "ui/message_center/message_center_util.h"
 
 // This file contains dummy implementation of MessageCenter and used to compile
 // and link with Android and iOS implementations of Chrome which do not have
 // notification systems yet. This is to avoid spreading compile-time flags
 // everywhere in the code.
 #if !defined(OS_ANDROID) && !defined(OS_IOS)
 #error This file should only be used in Android or iOS builds.
 #endif
 
 namespace message_center {
 
-bool IsRichNotificationEnabled() {
-  return false;
-}
-
 // static
 void MessageCenter::Initialize() {
 }
 
 // static
 MessageCenter* MessageCenter::Get() {
   return NULL;
 }
 
 // static
 void MessageCenter::Shutdown() {
 }
 
 MessageCenter::MessageCenter() {
 }
 
 MessageCenter::~MessageCenter() {
 }
 
 }  // namespace message_center