Use Spyder's prefs to enable/disable dark mode

[jitseniesen]

This is a follow-up from PR #396. Version 0.4 of the plugin has an option for choosing light or dark mode (in the Spyder plugin preferences). The implementation was very hacky and no longer works after #396. Users can now switch from light to dark using the Jupyter menu in the notebook. However, we still want the notebook to follow the Spyder preferences, so we should have the Spyder prefs overrule the setting in Jupyter and remove the menu entries for light/dark mode in the Jupyter menu.