block: remove the cmd_size field from struct request_queue

Entirely unused.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210920123328.1399408-11-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -544,8 +544,6 @@ struct request_queue {
 
 	bool			mq_sysfs_init_done;
 
-	size_t			cmd_size;
-
 #define BLK_MAX_WRITE_HINTS	5
 	u64			write_hints[BLK_MAX_WRITE_HINTS];
 };