Remove unused folder creation on file import

Part of backend#291 (closed): The file import task was able to both retrieve an existing folder from folder_id in the DataImport's payload and create a folder from element_name and element_type. However, element_name was never sent by the backend and element_type should be used for normal, non-folder elements.

This removes the auto-creation behavior, implicitly removing all mentions of element_name in a payload.