s390/xpram: Remove power management support

Power management support was removed for s390 with
commit 39421627 ("s390: remove broken hibernate / power management
support").

Remove leftover xpram-related power management code.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>