s390/net: Delete useless checks before function calls

The function debug_unregister() tests whether its argument is
NULL and then returns immediately. Thus the test around the call
is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>