How do I uninstall the Relation module?

Question

I am trying to uninstall the modules that are part of the Relation module, after I tested it on my test site.

The Relation Endpoints Field module is not selectable because Drupal says the field implemented by that module is being used.

When I click on the "Field list" link, I get the following list of fields currently used.

None of the reported fields is implemented by that module; they are standard Drupal fields.

What should I do, in such cases? The content type that used that field has been already deleted.