txn & producer: remove two TODOs

For txn, the TODO is dependent on the work being supported in Kafka: the
TODO has existed for a long time now and there's nothing that can be
done because Kafka is not supporting it yet.

For producer, not handling concurrent transactions in init producer ID
has never caused problems. It seems rare / likely to happen on client
initialization if it ever does.

pkg/kgo/producer.go

@@ -576,7 +576,7 @@ func (cl *Client) doInitProducerID(lastID int64, lastEpoch int16) (*producerID,
 	}
 
 	if err = kerr.ErrorForCode(resp.ErrorCode); err != nil {
-		if kerr.IsRetriable(err) { // TODO handle ConcurrentTransactions collision?
+		if kerr.IsRetriable(err) { // this could return ConcurrentTransactions, but this is rare; ignore until a user report
 			cl.cfg.logger.Log(LogLevelInfo, "producer id initialization resulted in retriable error, discarding initialization attempt", "err", err)
 			return &producerID{lastID, lastEpoch, err}, false
 		}

pkg/kgo/txn.go

@@ -608,7 +608,7 @@ func (cl *Client) maybeRecoverProducerID() (necessary, did bool, err error) {
 	}
 
 	kip360 := cl.producer.idVersion >= 3 && (errors.Is(ke, kerr.UnknownProducerID) || errors.Is(ke, kerr.InvalidProducerIDMapping))
-	kip588 := cl.producer.idVersion >= 4 && errors.Is(ke, kerr.InvalidProducerEpoch /* TODO || err == kerr.TransactionTimedOut */)
+	kip588 := cl.producer.idVersion >= 4 && errors.Is(ke, kerr.InvalidProducerEpoch /* || err == kerr.TransactionTimedOut when implemented in Kafka */)
 
 	recoverable := kip360 || kip588
 	if !recoverable {