Fix free meter widget when it's disabled

MeterContainer object tried to remove added objects in the overridden destroy()
method when GJS raised this error:
"Object .Gjs_SystemLoadItemsContainer (0x5589c3107050), has been already finalized. Impossible to set any property to it."

This commit fixes that by removing the custom destroy() method, but
remove the widget from the panel and call the original destroy() method.

System_Monitor@bghome.gmail.com/view.js

@@ -191,6 +191,8 @@ var Menu = new Lang.Class({
         if (this._meters[type]) {
             this._meters[type].removeObserver(meter_widget);
         }
+        this._widget_area_container.removeMeter(meter_widget);
+        meter_widget.destroy();
         delete this._meter_widgets[type];
     },
     destroy: function() {

System_Monitor@bghome.gmail.com/widget.js

@@ -306,16 +306,6 @@ const MeterContainer = new Lang.Class({
     },
     update: function(state) {
         this._label_item.setSummaryText(Math.round(state.percent) + ' %');
-    },
-    destroy: function() {
-        let actors = this.get_children();
-        for (let i = 0; i < actors.length; i++) {
-            let actor = actors[i];
-            this.remove_actor(actor);
-            actor.destroy();
-            actor = null;
-        }
-        this.parent();
     }
 });