quarantine: Always enable use of quarantine directories

Quarantine directories have been introduced for two reasons: first they are a correctness fix, where new objects don't end up in the target repo in case the request fails. And second, they allow us to speed up access checks enumerating new objects direcytl via the quarantine directory. Quarantine directories have been tested since August 9th in production now without any issues and can thus be deemed stable.

Remove the feature flags to make them generally available.

Changelog: performance

Closes #3734 (closed)