featureflag: Remove `AtomicFetch` feature flag
Atomic fetches have been tested in staging since last week and production since the beginning of this week without any issues at all.
Remove the feature flag to default-enable the feature. While this may feel fast, it was only added as a quick bailout in case it behaves different from what was expected. It didn't, so the removal should be safe.
Closes #3426 (closed)