block: remove outdated part of blkdev_issue_flush() comment

blkdev_issue_flush() is now always synchronous, and it no longer has a
flags argument.  So remove the part of the comment about the WAIT flag.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>