s390/mm: don't set ARCH_KEEP_MEMBLOCK

Commit 50be6345 ("s390/mm: Convert bootmem to memblock") mentions
	"The original bootmem allocator is getting replaced by memblock. To
	cover the needs of the s390 kdump implementation the physical
	memory list is used."

As we can now reference "physmem" managed in the memblock allocator after
init even without ARCH_KEEP_MEMBLOCK, and s390x does no longer need
other memblock metadata after boot (esp., the zcore memmap device that used
it got removed), we can stop setting ARCH_KEEP_MEMBLOCK.

With this change, we no longer create memblocks for standby/hotplugged
memory (added via add_memory()) and free up memblock metadata (except
physmem) after boot.

Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Philipp Rudo <prudo@linux.ibm.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200701141830.18749-3-david@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>