In this situation, the applicant left the U.S. prior to the original expiration of the I-94 and so, the request for extension is moot. However, there is no way of withdrawing an application once it is filed. You could, however, notify the USCIS of the departure and attach a copy of the stamp on the passport showing arrival abroad.
The USCIS will then confirm receipt of this letter and cancel the pending application for extension.