How to Hide the "Custom Login Page" Admin Menu

function plt_hide_err_custom_login_page_menus() {
	//Hide the "Settings → Custom Login Page" menu.
	remove_submenu_page('options-general.php', 'err-custom-login-settings');
}

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

Where do I put this code?