aio: remove an outdated BUG_ON and comment in aio_complete

These days we don't treat sync iocbs special in the aio completion code as
they never use it.  Remove the old comment and BUG_ON given that the
current definition of is_sync_kiocb makes it impossible to hit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>