uninstall.php

<?php
/**
 * Fired when the plugin is uninstalled.
 *
 * @package   Purify_WordPress_Menus
 * @author    Martin Stehle <m.stehle@gmx.de>
 * @license   GPL-2.0+
 * @link      http://wordpress.org/plugins/purify-wp-menues/
 * @copyright 2014 
 */

// If uninstall not called from WordPress, then exit
if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
	exit;
}

/**
* Delete options from the database while deleting the plugin files
* Run before deleting the plugin
*
* @since   2.0
*/
// remove settings
if ( is_multisite() ) {

	$sites = wp_get_sites();

	if ( empty ( $sites ) ) return;

	foreach ( $sites as $site ) {
		// switch to next blog
		switch_to_blog( $site[ 'blog_id' ] );
		// remove settings
		delete_option( 'widget_recent-posts-widget-with-thumbnails' );
	}
	// restore the current blog, after calling switch_to_blog()
	restore_current_blog();
} else {
	// remove settings
	delete_option( 'widget_recent-posts-widget-with-thumbnails' );
}