dm io: remove stale comment block for dm_io()

Commit 7eaceacc ("block: remove per-queue plugging") dropped
unplug_delay and blk_unplug(). Plus, the current kernel has no
fundamental difference between sync_io() and async_io() except
sync_io() uses sync_io_complete() as the notify.fn and explicitly
calls wait_for_completion_io() to sync. The comment isn't valid
any more.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-io.c

@@ -525,14 +525,6 @@ static int dp_init(struct dm_io_request *io_req, struct dpages *dp,
 	return 0;
 }
 
-/*
- * New collapsed (a)synchronous interface.
- *
- * If the IO is asynchronous (i.e. it has notify.fn), you must either unplug
- * the queue with blk_unplug() some time later or set REQ_SYNC in
- * io_req->bi_opf. If you fail to do one of these, the IO will be submitted to
- * the disk after q->unplug_delay, which defaults to 3ms in blk-settings.c.
- */
 int dm_io(struct dm_io_request *io_req, unsigned num_regions,
 	  struct dm_io_region *where, unsigned long *sync_error_bits)
 {