nvme-pci: remove stale comment

The ns variable has been removed in commit 62451a2b ("nvme: separate
command prep and issue"). Drop reference to ns in comment.

Fixes: 62451a2b ("nvme: separate command prep and issue")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>