remove DBDirectClient reference from ShardLocal::createIndexOnConfig

ShardLocal::createIndexOnConfig is called during primary step up as part of the config database initialization. Assuming that the config collection is always empty when this function is called, we should be able to replace this reference to DBDirectClient() with one of the new index creation helpers introduced in SERVER-43697. This will also eliminate another case where we build an index through the IndexBuildsCoordinator interface synchronously (SERVER-44436).