hw/ppc: Remove the deprecated spapr-pci-vfio-host-bridge device

It's a deprecated dummy device since QEMU v2.6.0. That should have
been enough time to allow the users to update their scripts in case
they still use it, so let's remove this legacy code now.

Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>