nvme: remove ns->disk checks

By the time a namespace is added to ctrl->namespaces list and thus
can be looked up ns->disk has been assigned, and it it never cleared.

Remove all the checks for ns->disk being NULL.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>