osd: Restyle OSD display popup

- remove class from label box
- new style for label itself

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2277>

js/ui/osdMonitorLabeler.js

@@ -13,7 +13,6 @@ class OsdMonitorLabel extends St.Widget {
         this._monitor = monitor;
 
         this._box = new St.BoxLayout({
-            style_class: 'osd-window',
             vertical: true,
         });
         this.add_actor(this._box);