xen service: fix wrong netmask handed out by xen-bridge.service

The dnsmasq instance run by the xen-bridge.service errorenously
hands out 172.16.0.0 as the netmask over DHCP to the VMs. This
commit removes the option responsible for that from dnsmasq.conf,
so that the proper netmask is inferred by dnsmasq instead.

Addresses https://github.com/NixOS/nixpkgs/issues/19883