s390: fix Oops when unloading module netiucv

don't remove an entry from iucv_connection_list in netiucv_exit().
netiucv_free_netdevice is called anyway, which takes care of entry
removal.

Signed-off-by: Ursula Braun <braunu@de.ibm.com>
Signed-off-by: Frank Pavlic <fpavlic@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>