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>