lib.types.string: Deprecation error instead of warning

The type has given a warning on use since [4 years ago](03392cd336b128a1639c648baf0f6c1a1271e0d2), I think it's safe to move the deprecation to the next stage: An error instead of a warning.

lib/types.nix

@@ -436,10 +436,7 @@ rec {
 
     # Deprecated; should not be used because it quietly concatenates
     # strings, which is usually not what you want.
-    string = separatedString "" // {
-      name = "string";
-      deprecationMessage = "See https://github.com/NixOS/nixpkgs/pull/66346 for better alternative types.";
-    };
+    string = throw "The type `types.string` is deprecated. See https://github.com/NixOS/nixpkgs/pull/66346 for better alternative types.";
 
     passwdEntry = entryType: addCheck entryType (str: !(hasInfix ":" str || hasInfix "\n" str)) // {
       name = "passwdEntry ${entryType.name}";