gnome-desktop: don't try to copy omf.make

* omf.make was removed from gnome-common in:
  https://git.gnome.org/browse/gnome-common/commit/?id=a912a74ba7999fe3ed49b07bab86685a5d117f82


  which is included in gnome-common-3.18, so this was broken since:
  commit f459f890cb7c0e11d92b80f0d5a65117428cc87f
  Author: Jussi Kukkonen <jussi.kukkonen@intel.com>
  Date:   Thu Nov 26 13:46:48 2015 +0200

    gnome-common: Upgrade 3.14.0 -> 3.18.0

  but hopefully we don't need omf.make with --disable-desktop-docs in EXTRA_OECONF

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>