Remove Image→DataFile link

This was supposed to allow telling which file an Image came from. The only API including this link is ListDataFile, and the datafile is actually never set by tasks when importing, so this link is simply never used. If it were ever used, there could be large performance issues with the IIIF import as a single JSON file would be linked to hundreds of images; and with IIIF, it would be possible for an image to be linked to two files, which this foreign key does not allow.