nixos/getty: revert overzealous assertion (#502957)

  ###### implementation

  config = mkIf config.console.enable {

    assertions = [

      {

        assertion = cfg.loginOptions != null -> cfg.autologinUser == null;

        message = "services.getty.autoLoginUser has no effect when services.getty.loginOptions is set.";

      }

    ];

    # Note: this is set here rather than up there so that changing

    # nixos.label would not rebuild manual pages

    services.getty.greetingLine = mkDefault ''<<< Welcome to ${config.system.nixos.distroName} ${config.system.nixos.label} (\m) - \l >>>'';