s390/ccwgroup: simplify ungrouping when driver deregisters

driver_unregister() ends up calling __device_release_driver() for
each device that is bound to the driver. Thus ccwgroup_notifier() will
see a BUS_NOTIFY_UNBIND_DRIVER event for these ccwgroup devices, and
trigger the ungrouping.

So there's no need to also trigger the ungrouping from within
ccwgroup_driver_unregister(), remove it.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>