How to Hide the "Visitors Right Now" Admin Menu

function plt_hide_visitors_right_now_uk_menus() {
	//Hide the "Tools → Visitors Right Now" menu.
	remove_submenu_page('tools.php', 'visitors');
}

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

Where do I put this code?