Reapply: Don't emit autoLogin config with eval warning

Fix this evaluation warning on the first rebuild after install:

  [test@nixos:~]$ sudo nixos-rebuild build
  building Nix...
  building the system configuration...
  evaluation warning: The option `services.xserver.displayManager.autoLogin' defined in `/etc/nixos/configuration.nix' has been renamed to `services.displayManager.autoLogin'.

Original commit id: 41f9b1b
Author: Bjørn Forsman <[email protected]>
Co-authored-by: Bjørn Forsman <[email protected]>

Author: FriedrichAltheide

modules/nixos/defaultConfigs/snippets/autologin.snippet

@@ -1,3 +1,3 @@
 # Enable automatic login for the user.
-services.xserver.displayManager.autoLogin.enable = true;
-services.xserver.displayManager.autoLogin.user = "@@username@@";
+services.displayManager.autoLogin.enable = true;
+services.displayManager.autoLogin.user = "@@username@@";