Remove cursor_mac.mm, fix GN build.

ui/base/cursor/cursor_mac.mm

Index: ui/base/cursor/cursor_mac.mm
diff --git a/ui/base/cursor/cursor_mac.mm b/ui/base/cursor/cursor_mac.mm
deleted file mode 100644
index 277717dad22fc33a037324b7f843fd28b238e852..0000000000000000000000000000000000000000
--- a/ui/base/cursor/cursor_mac.mm
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright 2014 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/base/cursor/cursor.h"
-
-namespace ui {
-
-void Cursor::RefCustomCursor() {
-  // TODO(macguru):
-}
-
-void Cursor::UnrefCustomCursor() {
-  // TODO(macguru):
-}
-
-}  // namespace ui