nixos/desktop-managers: do not leak feh to PATH

(originally from f9415cb621)

feh is used to set background image for desktop managers that do not
support it directly, however there is no need to include it in PATH.

Fixes #17450.

nixos/modules/services/x11/desktop-managers/default.nix

@@ -109,9 +109,5 @@ in
 
   };
 
-  config = {
-    services.xserver.displayManager.session = cfg.session.list;
-    environment.systemPackages =
-      mkIf cfg.session.needBGPackages [ pkgs.feh ]; # xsetroot via xserver.enable
-  };
+  config.services.xserver.displayManager.session = cfg.session.list;
 }