installer: use sddm in plasma5

Slim is abandoned and won't work with wayland.
It's in our best interest to use the display-manager
that makes most sense for Plasma5, sddm.

We've already moved on from it being default in NixOS#30890
and the graphical.nix profile, which the virtualbox profile uses,
has sddm anyway.

Author: worldofpeace

nixos/modules/installer/cd-dvd/installation-cd-graphical-base.nix

@@ -22,17 +22,7 @@ with lib;
     });
   '';
 
-  services.xserver = {
-    enable = true;
-
-    # Automatically login as nixos.
-    displayManager.slim = {
-      enable = true;
-      defaultUser = "nixos";
-      autoLogin = true;
-    };
-
-  };
+  services.xserver.enable = true;
 
   # Provide networkmanager for easy wireless configuration.
   networking.networkmanager.enable = true;

nixos/modules/installer/cd-dvd/installation-cd-graphical-gnome.nix

@@ -10,8 +10,6 @@ with lib;
 
   services.xserver.desktopManager.gnome3.enable = true;
 
-  services.xserver.displayManager.slim.enable = mkForce false;
-
   # Auto-login as root.
   services.xserver.displayManager.gdm.autoLogin = {
     enable = true;

nixos/modules/installer/cd-dvd/installation-cd-graphical-kde.nix

@@ -13,6 +13,15 @@ with lib;
       enable = true;
       enableQt4Support = false;
     };
+
+    # Automatically login as nixos.
+    displayManager.sddm = {
+      enable = true;
+      autoLogin = {
+        enable = true;
+        user = "nixos";
+      };
+    };
   };
 
   environment.systemPackages = with pkgs; [