Commit a7be6e5a ("mm: drop useless local parameters of
__register_one_node()") removes the last user of parent_node().

The parent_node() macro in S390 platform is unnecessary.

Remove it for cleanup.
Reported-by: NMichael Ellerman <mpe@ellerman.id.au>
Signed-off-by: NDou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>