Revert "nixos/mobilizon: fix integration test by using postgresql_14"

This reverts commit 80ba58361d92044864109b96f6ecef8e49e73f1d.

nixos/tests/mobilizon.nix

···
     meta.maintainers = with lib.maintainers; [ minijackson erictapen ];
 
     nodes.server =
-      { pkgs, ... }:
+      { ... }:
       {
         services.mobilizon = {
           enable = true;
···
             };
           };
         };
-
-        services.postgresql.package = pkgs.postgresql_14;
 
         security.pki.certificateFiles = [ certs.ca.cert ];