s390: remove xpram device driver

Support for expanded storage was only available until z13 and z/VM 6.3
respectively. However there haven't been any use cases a long time
before for this device driver.
Therefore remove it.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>