Cleanup Abandoned Database Rows Left Over from D5

This was brought to my attention in #310047: Field is not removed from shared content_field_XXX table on delete, where files that were deleted were leaving their data in the field tables. This means that sites upgraded from Drupal 5 are also likely have a large number of lingering entries in their field tables that don't actually match any entries in the files table.

This update sets those rows properly to NULL.