praefect: Drop feature flag that fixes routing with additional repos

When rewriting the target repository, the rewritten relative path will be specific to the node's storage to which the rewritten request is being routed to. Consequentially, if we're also rewriting an additional repository, it follows that both repositories must be hosted by the same storage, or otherwise one of both repositories wouldn't actually be resolveable on the local storage.

This property wasn't followed when routing repository creations that have an additional repository. This can lead to a case where we resolve the additional repository first, but ultimately assign the newly created repository to nodes that don't actually have the additional repository. The result is breakage.

We have fixed this bug in 41c10e01 (praefect/router: Fix creation of repos with additional referenced repo, 2023-05-10), where we started to route repository creating RPCs to the same nodes as the additional repository is assigned to. This change was rolled out more than a week ago into production systems without any observed issues.

Remove the feature flag guarding the code so that we unconditionally route such repository creating requests to the correct node.

Closes #5134 (closed).