objectpool: Default-enable pruning of refs to fix reference conflicts

In 018958fb (objectpool: Fix conflicting references when fetching into pools, 2022-07-27) we have introduced the logic to enable pruning of references when we update an object pool via FetchIntoObjectPool. This change was required because we have seen cases where updating object pools continued to fail due to conflicting references which exist in the pool repository and the member we have been fetching from. By pruning any references which don't exist in the member anymore we were able to resolve these conflicts and thus this bug.

This change has been rolled out to production on August 3rd without any observed issues. So let's default-enable the feature flag guarding this code so that the fix will be included in the next release.

Part of #4394 (closed)