Accept and ignore positional arguments shutdown

[mdegat01]

Fix this error on stop

Traceback (most recent call last):
Signal caught, exiting!
  File "/amr2mqtt/amr2mqtt.py", line 388, in main_loop
    amr_line = rtlamr.stdout.readline().strip()
TypeError: shutdown() takes 0 positional arguments but 2 were given
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/amr2mqtt/amr2mqtt.py", line 520, in <module>
    main_loop()
  File "/amr2mqtt/amr2mqtt.py", line 507, in main_loop
    time.sleep(2)
TypeError: shutdown() takes 0 positional arguments but 2 were given
Signal caught, exiting!