s390/topology: Remove the unused parent_node() macro

Commit a7be6e5a7f8d ("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: Michael Ellerman <[e-mail]>
Signed-off-by: Dou Liyang <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/include/asm/topology.h

···
 	return &node_to_cpumask_map[node];
 }
 
-/*
- * Returns the number of the node containing node 'node'. This
- * architecture is flat, so it is a pretty simple function!
- */
-#define parent_node(node) (node)
-
 #define pcibus_to_node(bus) __pcibus_to_node(bus)
 
 #define node_distance(a, b) __node_distance(a, b)