Drop `hide-markdown-diff` feature #4347

<!-- If you find any related threads, link them here -->

<!-- Duplicate issues are ok, but they may be closed pretty quickly -->

URL: https://github.com/sindresorhus/refined-github/edit/main/readme.md

Seems like GitHub implemented their own solution for this:

But maybe our feature can be extended to files other than .md ones? (Don't really see how that would be useful though)