That would make for quite a book if Harry turned out to be the Horcrux because in order to defeat Voldy he would have to kill himself or be killed by someone but if he is killed than there is no one to kill voldemort which means that evil would prevail and i doubt J.K would do that to everyone. That'd be awful if they killed off Harry
if Harry is the Horcrux they will find a way to extract it out of him without killing him hence he can finish off Voldy