xfs: remove dead ifdef XFSERRORDEBUG code

XFSERRORDEBUG is never set and the code isn't all that useful, so remove
it.

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>
Reviewed-by: Brian Foster <bfoster@redhat.com>