[S390] xpram system device class.

Remove system device class for xpram. It creates the directory hierarchy
under /sys/devices/system/xpram/xpram0. The xpram0 directory is empty and
it is always created while xpram1 and following devices are always missing,
independent if the devices exist or not. Since the xpram devices are
listed in /proc/partitions and /sys/block/ as slram<x> the system device
class for xpram is meaningless.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>