You could answer this question in a couple of different ways.
First, there is clearly someone who was present when King Laius was killed and who survived the attack. We are not told exactly who the person is, only that he was either a messenger or a fellow traveler. The person said that a large number of bandits attacked King Laius and his people and killed them.
Another way to answer the question is to say that there was no real witness. The person who was there saw what happened but essentially knows (or will admit) nothing. He says that he can tell them nothing other than that bandits were responsible. He did not see who they were or anything like that. So in that sense, you could say that there is no useful witness.