xfs: remove IO_ISAIO

We set the IO_ISAIO flag for all read/write I/O since early Linux
2.6.x.  Remove it as it has lost it's purpose long ago.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Alex Elder <aelder@sgi.com>