Remove opinionated `<sub>` and `<sup>` styles

I suspect that these styles come from the normalize.css reset. They were removed in 2017: Remove sub and sup opinionated resets · csstools/normalize.css@1880b36 because "they are definitely not normalizations in the sense of polyfills, but rather opinionated defaults": https://github.com/csstools/sanitize.css/issues/175#issuecomment-445109735

Because of that, I think it might make sense to remove them from modern-normalize.

Maybe add them in #13 (modern-base)?