nixos/xss-lock: remove startx warning

This warning is based on a misconception: xss-lock, as most user
services, just require access to the shell environment variables,
which for `startx` have to be imported manually.

nixos/modules/programs/xss-lock.nix

@@ -49,10 +49,5 @@ in
       );
       serviceConfig.Restart = "always";
     };
-
-    warnings = lib.mkIf (config.services.xserver.displayManager.startx.enable) [
-      "xss-lock service only works if a displayManager is set; it doesn't work when services.xserver.displayManager.startx.enable = true"
-    ];
-
   };
 }