workspace-indicator: Don't override ClutterActor.destroy()

Now that PanelMenu.Button was made an StWidget subclass, the destroy()
method actually maps to the ClutterActor method, and overriding it
results in warnings when the extension is disabled. So instead, use
the existing ::destroy handler.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/113

extensions/workspace-indicator/extension.js

@@ -59,7 +59,7 @@ class WorkspaceIndicator extends PanelMenu.Button {
                                    this._createWorkspacesSection.bind(this));
     }
 
-    destroy() {
+    _onDestroy() {
         for (let i = 0; i < this._workspaceManagerSignals.length; i++)
             global.workspace_manager.disconnect(this._workspaceManagerSignals[i]);
 
@@ -68,7 +68,7 @@ class WorkspaceIndicator extends PanelMenu.Button {
             this._settingsChangedId = 0;
         }
 
-        super.destroy();
+        super._onDestroy();
     }
 
     _updateIndicator() {