Remove field timeout of Session

For now it seems timeout in Session is just for a minimum value of timeout. I don't think we should hard code the minimum value of timeout as it would make it difficult to do session timeout testing. I think we can just make 1 hour as the default timeout and user can set whatever value they want through livy.server.session.timeout

    def expired(session: Session): Boolean = {
      val currentTime = System.nanoTime()
      currentTime - session.lastActivity > math.max(sessionTimeout, session.timeout)
    }