desktop-managers: do not leak feh to PATH

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
-  };
+  xcfg.displayManager.session = cfg.session.list;
 }