ahci: kill leftover from enabling NCQ over PMP

ahci had problems with NCQ over PMP and NCQ used to be disabled while
PMP was attached.  After fixing the problem, the temporary NCQ
disabling code wasn't removed completely.  Kill the remaining piece.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>