CollectionImpl::insertDocumentForBulkLoader perform oplog writes  and this function is called only by CollectionBulkLoaderImpl::insertDocuments. CollectionBulkLoader is only used by initial sync collection cloner which should not perform oplog writes during collection writes. So, its good to remove the unsed oplog write code block to avoid confusion and add an invariant to check that oplog writes are disabled when insertDocumentForBulkLoader() is called.