Put a brake on. Riding brakeless looks cool (which is pretty much the only upside there is to riding brakeless), until you slam into the back of a car.
Riding brakeless probably isn't enough to kill your knees if you avoid excessive backpedaling and skidding, and have the bike correctly fitted, but you are much more likely to kill your knees in a crash by tearing your ACL or something similar.
Riding brakeless also puts other people at risk. A guy in the UK was sent to jail because he killed a pedestrian while riding brakeless.