nixos/tests: clean up pgjwt test

- removed unneeded initscript
- use default postgres version for the test

nixos/tests/pgjwt.nix

@@ -18,12 +18,7 @@ with pkgs; {
     {
       services.postgresql = {
         enable = true;
-      package = postgresql96;
         extraPlugins = [ pgjwt pgtap ];
-      initialScript =  writeText "postgresql-init.sql"
-      ''
-        CREATE ROLE postgres WITH superuser login createdb;
-      '';
       };
     };
   };