block: remove obsolete comments for blkdev_issue_zeroout.

barrier is already removed, so remove the obsolete comments
in blkdev_issue_zeroout.

Cc: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-lib.c

@@ -140,8 +140,6 @@ static void bio_batch_end_io(struct bio *bio, int err)
  *
  * Description:
  *  Generate and issue number of bios with zerofiled pages.
- *  Send barrier at the beginning and at the end if requested. This guarantie
- *  correct request ordering. Empty barrier allow us to avoid post queue flush.
  */
 
 int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,