How to Hide the "Mark New Posts" Admin Menu

function plt_hide_mark_new_posts_menus() {
	//Hide the "Settings → Mark New Posts" menu.
	remove_submenu_page('options-general.php', 'mark-new-posts.php');
}

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

Where do I put this code?