fix(preview-cleanup): Also delete previews stored in the oc_previews table

[CarlSchwan]

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [x] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)
• [x] Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• [x] Milestone added for target branch/version (ex: 32.x for stable32)

[CarlSchwan]

Not sure about the transaction question: right now we don't use transactions, right?

Yeah, something we might want to reconsider. But for later :)