How to Hide the "Woostify Sites Library" Admin Menu

function plt_hide_woostify_sites_library_menus() {
	//Hide the "Appearance → Woostify Sites Library" menu.
	remove_submenu_page('themes.php', 'woostify-sites');
}

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

Where do I put this code?