Fix exception raised when closing preferences

[Yutsuten]

Regression from #53 .
I was unable to close Anki in the usual way. Had to kill the process.

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/aqt/preferences.py", line 48, in reject
    self.accept()
  File "<decorator-gen-10>", line 2, in accept
  File "/usr/lib/python3.8/site-packages/anki/hooks.py", line 638, in decorator_wrapper
    return repl(*args, **kwargs)
  File "/usr/lib/python3.8/site-packages/anki/hooks.py", line 632, in repl
    new(*args, **kwargs)
  File "/home/mateus/.local/share/Anki2/addons21/lifedrain/main.py", line 48, in <lambda>
    Preferences.accept, lambda *args: lifedrain.preferences_save(args[0]),
  File "/home/mateus/.local/share/Anki2/addons21/lifedrain/lifedrain.py", line 77, in preferences_save
    conf = self._settings.preferences_save(pref)
  File "/home/mateus/.local/share/Anki2/addons21/lifedrain/settings.py", line 183, in preferences_save
    conf.pop('barBgColor', None)
AttributeError: 'ConfigManager' object has no attribute 'pop'