Remove implicit pool creation on link behavior

If a repository is being linked to an object pool that does not exist, the current behavior is to create the pool implicitly. This doesn't bode well with Praefect as it doesn't know whether a pool was or wasn't created and can't thus create a metadata record for the pool. The functionality was removed behind a feature flag. It doesn't seem like anything relies on the implicit pool creation, so this MR removes the feature flag and returns an error when linking a repository to a non-existent pool.

Part of #4106 (closed)