code: public boolean unlink(Object source, String attributeName, Object target)
      {
          return linkOrUnlink(false, source, attributeName, false);
      }
docs: Unlink the specified reference object.
  More info see OJB doc.
  @param source The source object with the specified reference field.
  @param attributeName The field name of the reference to unlink.
  @param target The referenced object to unlink.