I think that maybe there is. Their testimony has to be in their capacity as a police officer. You wouldn't have a doctor who lost his/he medical license testify as an expert witness. You wouldn't have a disbarred lawyer testify as to the law, etc.How does his subsequent arrested bar him from testifying to the events?
Of course, bringing a (possibly) discredited officer in to testify has its downsides, but there is nothing that would prevent the state from doing so.