DeleteFolder method
Implementation
Future<void> DeleteFolder(FolderId? folderId, DeleteMode deleteMode) {
  EwsUtilities.ValidateParam(folderId, "folderId");
  DeleteFolderRequest request =
      new DeleteFolderRequest(this, ServiceErrorHandling.ThrowOnError);
  request.FolderIds.AddFolderId(folderId);
  request.DeleteMode = deleteMode;
  return request.Execute();
}