Remove update_checker

[peterbarker]

Doesn't work on Python3 and would probably annoy more people than it helps if it was working.

Traceback (most recent call last):
  File "/home/pbarker/.local/lib/python3.10/site-packages/dronecan_gui_tool/update_checker.py", line 157, in do_background_check
    update_reference = _do_pip_check()
  File "/home/pbarker/.local/lib/python3.10/site-packages/dronecan_gui_tool/update_checker.py", line 84, in _do_pip_check
    data = json.loads(data.decode('utf8'), encoding='utf8')
  File "/usr/lib/python3.10/json/__init__.py", line 359, in loads
    return cls(**kw).decode(s)
TypeError: JSONDecoder.__init__() got an unexpected keyword argument 'encoding' 2024-01-12 11:27:37,946 INFO dronecan_gui_tool.update_checker Update checker stopped