Batch item importers can't unlink a data file from the owning item

[Nicklas Nordborg]

The batch importers should be able create, modify and delete links to files. It seems like it doesn't work to delete the link.

A simple test file can be created with the following columns:

• Name: The name of the owning item
• Data file type: The name of the file type to remove
• File: Keep this column empty

Add data to the test file and make sure that at least one item already has a link to a file of the specified file type.

When running the batch importer it claims to have update a number of items, but it seems like nothing has changed.

[Nicklas Nordborg]

In 8167:

Fixes #2308: Batch item importers can't unlink a data file from the owning item

The problem was a test that was reversed TRUE/FALSE.