nvme: remove a misleading comment on struct nvme_ns

While a NVMe Namespace is somewhat similar to a SCSI Logical Unit (and not
a Logical Unit Number anyway) there are subtile differences.  Remove the
misleading comment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grmberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>