How to Hide the "Cleanup Database" Admin Menu

function plt_hide_plugins_garbage_collector_menus() {
	//Hide the "Tools → Cleanup Database" menu.
	remove_submenu_page('tools.php', 'plugins-garbage-collector');
}

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

Where do I put this code?