fixed DDRFile.file_path format string

There were no errors so apparently this isn't being used.
remove entity files dir name from file relative paths

Instead of listing
  'files/ddr-testing-1-1-master-a1b2c3.jpg'
in entity.json and file .json, list
  'ddr-testing-1-1-master-a1b2c3.jpg'
This is consistent with how it's done in collection metadata
and will make it possible for the 'files/' directory name to
become a setting in future without too much disruption.
rm extra access_rel field from file JSON