Unregisters a :class:`~horizon.Panel` from this dashboard.

def unregister(cls, panel):
        """Unregisters a :class:`~horizon.Panel` from this dashboard."""
        success = Horizon.unregister_panel(cls, panel)
        if success:
            # Remove the panel's template directory.
            key = os.path.join(cls.slug, panel.slug)
            if key in loaders.panel_template_dirs:
                del loaders.panel_template_dirs[key]
        return success