osx: Remove legacy extras/MacOSX directory

Backport of http://review.gluster.org/8736

BUG: 1141683
Change-Id: Ic37df769db856196727d2799396c1dbaf4bcdd1a
Signed-off-by: Justin Clift <justin@gluster.org>
Reviewed-on: http://review.gluster.org/8737
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>