How to Hide the "Regenerate Thumbnails" Admin Menu

function plt_hide_regenerate_thumbnails_menus() {
	//Hide the "Tools → Regenerate Thumbnails" menu.
	remove_submenu_page('tools.php', 'regenerate-thumbnails');
}

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

Where do I put this code?