block: remove an outdated comment on the bd_dev field

kdev_t is long gone, so we don't need to comment a field isn't one..
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blk_types.h

@@ -20,7 +20,7 @@ typedef void (bio_end_io_t) (struct bio *);
 struct bio_crypt_ctx;
 
 struct block_device {
-	dev_t			bd_dev;  /* not a kdev_t - it's a search key */
+	dev_t			bd_dev;
 	int			bd_openers;
 	struct inode *		bd_inode;	/* will die */
 	struct super_block *	bd_super;