theon: stop scripting networking

Unsure why this host is using systemd-networkd, but leave that unchanged
and have NixOS know about it to prevent a warning about loss of
connectivity on build.

hosts/theon.storage.ts.hillion.co.uk/default.nix

@@ -22,6 +22,7 @@
     };
 
     ## Networking
+    networking.useNetworkd = true;
     systemd.network.enable = true;
 
     networking.nameservers = lib.mkForce [ ]; # Trust the DHCP nameservers