block: remove NFL4_UFLG_MASK

The NFL4_UFLG_MASK define slipped in in commit 9208d414
("block: add a ->get_unique_id method") and should never have been
added, as NFSD as the only user of it already has it's copy.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230520090010.527046-1-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -1376,8 +1376,6 @@ enum blk_unique_id {
 	BLK_UID_NAA	= 3,
 };
 
-#define NFL4_UFLG_MASK			0x0000003F
-
 struct block_device_operations {
 	void (*submit_bio)(struct bio *bio);
 	int (*poll_bio)(struct bio *bio, struct io_comp_batch *iob,