[XFS] remove dmapi cruft in xfs_file.c

The dmapi cruft in xfs_file.c is totally out of date in mainline vs
CVS, and at this point just removing this code which can't be used on
mainline at all seems to be the best option to keep it maintainable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>