s390x virtio-ccw machine: step down as maintainer

I currently don't have time to work on the s390x virtio-ccw machine
anymore, so let's step down. (I will, however, continue as a
maintainer for the virtio-ccw *transport*.)

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Message-Id: <20211012144040.360887-4-cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>