After having spent 15 minutes looking for a way of deleting the toolbar, I restored and old version of the settings, copied the File button, restored the latest settings, and pasted the File button.
As Leo wrote, If you delete the ListerMenu toolbar and restart Opus then the default toolbar will be restored. BUT: If the toolbar wasn't enabled, it is still NOT enabled. You have to click customize -> toolbars and enable the restored default toolbar.