Don't show file upload when 'Supply path directly' is selected

When the file fetcher's setting 'supply path directly' setting is selected, the file upload is still shown. WIth recent changes to the fetcher where Feeds uses the File API to track files that are uploaded, but don't track files that are merely supplied by path this gets rather confusing. For instance, files that are uploaded get deleted with the source. Files that supplied by path reference are never touched.

Let's hide the file upload when the 'supply path directly' feature is chosen. Also cleans up the form tremendously.