dialog: Remove addContent function

Now that child metas are removed, there's no need for a custom
`addContent` function anymore. It was only used in one place anyway...

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/886

js/ui/closeDialog.js

@@ -67,7 +67,7 @@ var CloseDialog = GObject.registerClass({
         this._dialog.width = windowActor.width;
         this._dialog.height = windowActor.height;
 
-        this._dialog.addContent(this._createDialogContent());
+        this._dialog.contentLayout.add_child(this._createDialogContent());
         this._dialog.addButton({ label: _('Force Quit'),
                                  action: this._onClose.bind(this),
                                  default: true });