s390/virtio: remove the old KVM virtio transport

There is no recent user space application available anymore which still
supports this old virtio transport. Additionally, commit 3b2fbb3f
("virtio/s390: deprecate old transport") introduced a deprecation message
in the driver, and apparently nobody complained so far that it is still
required. So let's simply remove it.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>