window-list: Fix cleaning up signal on removal

The map uses windows as key, so trying to remove the handler ID
will leave stray windows/signals.

Spotted by Ron Yorston.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/225>

extensions/window-list/extension.js

@@ -998,7 +998,7 @@ class WindowList extends St.Widget {
         const id = this._windowSignals.get(win);
         if (id)
             win.disconnect(id);
-        this._windowSignals.delete(id);
+        this._windowSignals.delete(win);
 
         let children = this._windowList.get_children();
         let child = children.find(c => c.metaWindow === win);