s390x/css: remove unused error handling branch

The case in question actually never happens. Let us get rid of the dead
code.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Message-Id: <20170908152446.14606-4-pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>