f2fs: remove redundant comment of unused wio_mutex

Commit 089842de ("f2fs: remove codes of unused wio_mutex") removes codes
of unused wio_mutex, but missing the comment, so delete it.
Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -1175,7 +1175,6 @@ struct f2fs_sb_info {
 
 	/* for bio operations */
 	struct f2fs_bio_info *write_io[NR_PAGE_TYPE];	/* for write bios */
-						/* bio ordering for NODE/DATA */
 	/* keep migration IO order for LFS mode */
 	struct rw_semaphore io_order_lock;
 	mempool_t *write_io_dummy;		/* Dummy pages */