theme: Drop custom assets for window close buttons in overview

They can be replaced by a themed icon and some CSS styling.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/782

js/ui/workspace.js

@@ -473,6 +473,7 @@ var WindowOverlay = new Lang.Class({
         });
 
         let button = new St.Button({ style_class: 'window-close' });
+        button.add_actor(new St.Icon({ icon_name: 'window-close-symbolic' }));
         button._overlap = 0;
 
         this._idleToggleCloseId = 0;