panelMenu: Remove SystemIndicator class

It was used by components in the aggregate menu. That menu no
longer exists, and the class has been superseded by the class of
the same name in quick settings.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3068>

js/ui/panelMenu.js

@@ -195,41 +195,3 @@ export const Button = GObject.registerClass({
         super._onDestroy();
     }
 });
-
-/* SystemIndicator:
- *
- * This class manages one system indicator, which are the icons
- * that you see at the top right. A system indicator is composed
- * of an icon and a menu section, which will be composed into the
- * aggregate menu.
- */
-export const SystemIndicator = GObject.registerClass(
-class SystemIndicator extends St.BoxLayout {
-    _init() {
-        super._init({
-            style_class: 'panel-status-indicators-box',
-            reactive: true,
-            visible: false,
-        });
-        this.menu = new PopupMenu.PopupMenuSection();
-    }
-
-    get indicators() {
-        let klass = this.constructor.name;
-        let {stack} = new Error();
-        log(`Usage of indicator.indicators is deprecated for ${klass}\n${stack}`);
-        return this;
-    }
-
-    _syncIndicatorsVisible() {
-        this.visible = this.get_children().some(a => a.visible);
-    }
-
-    _addIndicator() {
-        let icon = new St.Icon({style_class: 'system-status-icon'});
-        this.add_child(icon);
-        icon.connect('notify::visible', this._syncIndicatorsVisible.bind(this));
-        this._syncIndicatorsVisible();
-        return icon;
-    }
-});