[S390] qdio: remove multicast polling

The multicast poll check for the outbound queue is redundant since
3d6c76ff "[S390] qdio: outbound tasklet scan threshold". Remove the check.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>