Remove Darwin CMake file

The file is now empty and thus serves no purpose.

cmake/os/Darwin.cmake

-# Copyright (c) 2010, 2014, Oracle and/or its affiliates. All rights reserved.
-# 
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation; version 2 of the License.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program; if not, write to the Free Software
-# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1335  USA 
-
-# This file includes OSX specific options and quirks, related to system checks