windowPreview: Keep borderCenter always visible

We need it to position the icon now, which isn't hidden with the
border.

https://gitlab.gnome.org/Teams/Design/os-mockups/-/issues/81

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1605>

js/ui/windowPreview.js

@@ -297,8 +297,6 @@ var WindowPreview = GObject.registerClass({
             factor: 0.5,
         }));
         this._borderCenter = new Clutter.Actor();
-        this._border.bind_property('visible', this._borderCenter, 'visible',
-            GObject.BindingFlags.SYNC_CREATE);
         this._borderCenterConstraint = new Clutter.BindConstraint({
             source: this._windowContainer,
             coordinate: Clutter.BindCoordinate.SIZE,