xfs: Remove struct xfs_chash from xfs_mount

Remove struct xfs_chash from struct xfs_mount as there is no user of
it nowadays.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>