nixos/desktop-managers: Remove needBGPackages thing

This value isn't used since 71a8dbb956f5735030cd3982263d72f1bffdae23

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

@@ -68,8 +68,7 @@ in
           scripts before forwarding the value to the
           <varname>displayManager</varname>.
         '';
-        apply = list: {
-          list = map (d: d // {
+        apply = map (d: d // {
           manage = "desktop";
           start = d.start
           + optionalString (needBGCond d) ''
@@ -80,9 +79,7 @@ in
               ${pkgs.xorg.xsetroot}/bin/xsetroot -solid black
             fi
           '';
-          }) list;
-          needBGPackages = [] != filter needBGCond list;
-        };
+        });
       };
 
       default = mkOption {
@@ -100,5 +97,5 @@ in
 
   };
 
-  config.services.xserver.displayManager.session = cfg.session.list;
+  config.services.xserver.displayManager.session = cfg.session;
 }