refactoring request: removing `(N)SfwCategory` enum

splitting the category into `NsfwCategory` and `SfwCategory` is too
- complicated (casting between enums etc) and
- repetitive (same code for each enum) and
- hard to maintain.

and it is also too feature-overusing.
if necessary, it might be better to use `#[cfg(feature ..)]` to only specific variants so that there is only one enum.
thus this is actually not a good idea,
so if you don't mind, could I send a PR refactoring this problem?

feel free to tell me your opinion on my suggestion if you are okay!