nbd: remove nbd_del_disk

Fold nbd_del_disk and remove the pointless NULL check on ->disk given
that it is always set for a successfully allocated nbd_device structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210811124428.2368491-4-hch@lst.de
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>