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

chrome/browser/notifications/balloon_collection.cc

Index: chrome/browser/notifications/balloon_collection.cc
===================================================================
--- chrome/browser/notifications/balloon_collection.cc	(revision 263121)
+++ chrome/browser/notifications/balloon_collection.cc	(working copy)
@@ -1,12 +0,0 @@
-// Copyright (c) 2011 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 "chrome/browser/notifications/balloon_collection.h"
-
-BalloonCollection::BalloonCollection()
-    : space_change_listener_(NULL) {
-}
-
-BalloonCollection::~BalloonCollection() {
-}