[S390] cpumask: remove cpu_coregroup_map

Impact: cleanup

cpu_coregroup_mask is the New Hotness.

As S/390 uses theirs internally, so we just make it static.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>