objectpool: Always reject fetching into nonexistent pools

With 2d2023a4 (objectpool: Reintroduce error if pool does not exist on fetch, 2022-11-04), we have introduced a feature flag that causes us to fail when FetchIntoObjectPool is executed with an object pool that does not exist. This is done to fix multiple issues:

- The lifetime of object pools is not clearly scoped, which makes it
  hard to reason about them.

- Depending on whether an object pool has been created via
  CreatePool or via FetchIntoObjectPool, their state will be
  different.

- Praefect would fail to properly handle object pools as the RPC is
  not marked as a repository-creating one.

We have rolled out these changes to production a week ago without any negative consequences observed so far. So let's remove the feature flag altogether to unconditionally fix all of the above issues.

Changelog: fixed

Closes #4560 (closed).