meta(home-manager): Determine isPhone from nixosConfig

Simplifies things a bit since we no longer depend on a phone variable,
thus making the individual files easier to copy/paste.

modules/system.nix

@@ -189,7 +189,7 @@ in
       };
 
       extraSpecialArgs = {
-        inherit nix-config phone;
+        inherit nix-config;
       };
     };