gfs2: Remove gfs2_xattr_acl_chmod

Function gfs2_xattr_acl_chmod is unused since commit e01580bf.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Acked-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>