Rather than delete an relationship type, you can mark it as inactive. The relationship type remains in your database, but users can no longer apply it to constituent relationships. To mark a relationship type as inactive, select it on the Relationship Types tab on the Relationship Settings page and click Mark inactive. To reactivate an inactive relationship type, select it under Relationship types and click Mark active.
To display inactive relationship types on the Relationship Types page, click Filter, select Include inactive, and click Apply.