How to Hide the "Import Demo Data" Admin Menu

function plt_hide_one_click_demo_import_menus() {
	//Hide the "Appearance → Import Demo Data" menu.
	remove_submenu_page('themes.php', 'one-click-demo-import');
}

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

Where do I put this code?