Completes the asynchronous operation started in BeginDeleteDirectoryTree.
Before calling BeginDeleteDirectoryTree, you need to create a callback method that implements the AsyncCallback delegate. This callback method executes in a separate thread and is called by the system after BeginDeleteDirectoryTree returns. The callback method must accept the IAsyncResult returned by the BeginDeleteDirectoryTree method as a parameter. This parameter may then be passed to the EndDeleteDirectory method to complete the operation.