direct-io: Remove unused DIO_ASYNC_EXTEND flag

This flag was added by 60392573 ("direct-io: add flag to allow aio
writes beyond i_size") to support XFS. However, with the rework of
XFS' DIO's path to use iomap in acdda3aa ("xfs: use iomap_dio_rw")
it became redundant. So let's remove it.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>