If you want to delete an additional field, you must first call the additional information screen for the corresponding database table. To do this, place the cursor on the additional field and choose Delete.
This is possible only if the additional field is not used by any queries. If you want to delete an additional field, even if it is used in a query, you must first change all the queries affected. To do this, follow the steps described in Deleting the Assignment of Additional Tables.