xfs: remove the never fully implemented UUID fork format

Remove the dead code dealing with the UUID fork format that was never
implemented in Linux (and neither in IRIX as far as I know).

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