s390/virtio: remove unused pm callbacks

s390/virtio: remove unused pm callbacks

Support for hibernation on s390 has been recently been removed with
commit 394216275c7d ("s390: remove broken hibernate / power management
support"), no need to keep unused code around.

Link: https://lkml.kernel.org/r/20200526093629.257649-1-cohuck@redhat.com
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>