tests/minimal: drop

We have other tests that are minimal, e.g. the multiple test. And this
test wasn't even hooked up in flake.nix, so I'm doubtful that we really
need it.

tests/minimal.nix

@@ -1,30 +0,0 @@
-#  nixos-mailserver: a simple mail server
-#  Copyright (C) 2016-2018  Robin Raymond
-#
-#  This program is free software: you can redistribute it and/or modify
-#  it under the terms of the GNU General Public License as published by
-#  the Free Software Foundation, either version 3 of the License, or
-#  (at your option) any later version.
-#
-#  This program is distributed in the hope that it will be useful,
-#  but WITHOUT ANY WARRANTY; without even the implied warranty of
-#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-#  GNU General Public License for more details.
-#
-#  You should have received a copy of the GNU General Public License
-#  along with this program. If not, see <http://www.gnu.org/licenses/>
-
-{
-  name = "minimal";
-
-  nodes.machine = {
-    imports = [
-      ../default.nix
-      ./lib/config.nix
-    ];
-  };
-
-  testScript = ''
-    machine.wait_for_unit("multi-user.target");
-  '';
-}