gnome.gnome-panel: Use autoconf271

I don't see any build issues with it.

pkgs/desktops/gnome/default.nix

···
 
   gnome-flashback = callPackage ./misc/gnome-flashback { };
 
-  gnome-panel = callPackage ./misc/gnome-panel {
-    autoreconfHook = pkgs.autoreconfHook269;
-  };
+  gnome-panel = callPackage ./misc/gnome-panel { };
 
   gnome-tweaks = callPackage ./misc/gnome-tweaks { };