objectpool: Always prune refs on fetches to fix D/F conflicts

With 018958fb (objectpool: Fix conflicting references when fetching into pools, 2022-07-27) we have started to prune references in objetc pools that have been removed in their primary pool member. This is a necessity to avoid conflicting references in the object pool and its member, which would otherwise break updating such pools completely.

This code has been rolled out to production on August 11th without any observed negative fallout, and the flag was subsequently enabled by default. Remove the feature flag guarding it.

Changelog: changed

Closes #4394 (closed)