s390/cio: dont unregister subchannel from child-drivers

The devices owned by the parent-driver (css) was getting unregistered
from the io-subchannel driver is clearly a layering violation.
Remove the subchannel unregistration from the child-drivers.

This also means, if the device connected to the subchannel is not
operational, or not accessible, the subchannel will not be unregistered.
Instead the CIO layer will allow valid subchannels without any operational
devices in sysfs. And the userspace tooling might need to be modified to
optimally handle this new situation.

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>