osx: Remove legacy extras/MacOSX directory

BUG: 1141682
Change-Id: Ic84171b070985a11b70d4548aa5e507a121d2164
Signed-off-by: Justin Clift <justin@gluster.org>
Reviewed-on: http://review.gluster.org/8736
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Gluster Build System <jenkins@build.gluster.com>