How to Hide the "Disk Usage" Admin Menu

function plt_hide_disk_usage_sunburst_menus() {
	//Hide the "Tools → Disk Usage" menu.
	remove_submenu_page('tools.php', 'disk-usage-sunburst/rbdusb-disk-usage-sunburst.php');
}

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

Where do I put this code?