nixos/slim: remove

The SLIM project is abandoned and their last release was in 2013.
Because of this it poses a security risk to systems, no one is working
on it or picked up maintenance. It also lacks compatibility with systemd
and logind sessions. For users, there liikely isn't anything like slim
that's as lightweight in terms of dependencies.

nixos/lib/testing-python.nix

@@ -258,7 +258,6 @@ in rec {
           inherit require;
           virtualisation.memorySize = 1024;
           services.xserver.enable = true;
-          services.xserver.displayManager.slim.enable = false;
           services.xserver.displayManager.auto.enable = true;
           services.xserver.windowManager.default = "icewm";
           services.xserver.windowManager.icewm.enable = true;