message: remove old heartbeat function
diff: @@ -25,15 +25,6 @@ class ParState(Enum):
  DONE = auto()
  -# deprecated: see message_loop
  -def heartbeat(channel, terminate_event):
  - stub = coordinator_pb2_grpc.CoordinatorStub(channel)
  - while not terminate_event.is_set():
  - reply = stub.Heartbeat(coordinator_pb2.HeartbeatRequest())
  - print(f"Participant received: {type(reply)}")
  - time.sleep(HEARTBEAT_TIME)
  -
  -
  def rendezvous(channel):
  """Starts a rendezvous exchange with Coordinator.
  