How to Hide the "Simple Image XML Sitemap" Admin Menu

function plt_hide_simple_image_xml_sitemap_menus() {
	//Hide the "Tools → Simple Image XML Sitemap" menu.
	remove_submenu_page('tools.php', 'simple-xml-image-sitemap');
}

add_action('admin_menu', 'plt_hide_simple_image_xml_sitemap_menus', 11);

Where do I put this code?