removed unused Common import

Author: Maximilian Taube

Validator.php

@@ -10,7 +10,6 @@
 namespace Piwik\Plugins\CustomAlerts;
 
 use Exception;
-use Piwik\Common;
 use Piwik\Context;
 use Piwik\Piwik;
 use Piwik\Plugins\API\ProcessedReport;