s390/cio: remove pm support from css-bus driver

The power-management functions are unused since the
'commit 39421627

 ("s390: remove broken hibernate / power
management support")'. Remove the unused pm callbacks from
css-bus driver.

Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>