phone: Remove mullvad-vpn service due to missing rpfilter

The mullvad-vpn service depends on iptables, which breaks support on the
PinePhone.

phone.nix

@@ -11,13 +11,9 @@ in
 
   modules = {
     system = {
-      mullvad = true;
+      phone = true;
       hostName = "mobile-nixos";
       stateVersion = "23.11";
     };
-
-    desktop = {
-      phone = true;
-    };
   };
 }