Add support for removing obsolete config options from the file

[NorbiPeti]

Setting a config option's value to null could be used to remove that value, however, the value will be set to the default the next time the plugin is enabled. Removing nullability even for setting the value helps with unexpected behaviors.

The only use I can think of right now for removing a config option is to remove obsolete ones. Since the config is automatically generated, there's no use in removing one that matches the default or something.

Here's another idea: Maybe all unused paths should be removed from the config. But then we'd need to know each option that is used by the plugin.