s390/sclp: Remove sclp base power management support

Power management support was removed for s390 with
commit 39421627

 ("s390: remove broken hibernate / power management
support").

Remove leftover sclp base-related power management code. Note that we
keep the registration of the sclp platform driver since it is used to
externalize non-PM related attributes in sysfs.

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>