Remove special case for darwin in ns_disksup.

This is already fixed in R16.

Change-Id: Ib11f61199213b012fd3b79da6dc20ae002c789a4
Reviewed-on: http://review.couchbase.org/38455
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>