In SERVER-47844 we added the non-const virtual method KVEngine::getInitialDataTimestamp. Strangely, its subclass WiredTigerKVEngine already had a const non-virtual method of the same name. In SERVER-48518 we removed the const non-virtual WiredTigerKVEngine::getInitialDataTimestamp, so it had only its non-const virtual override. Let's clean up after ourselves: getInitialDataTimestamp should be a const virtual method.