deprecate allow_dark_preference_with_rfp.patch

I stopped including this patch a while ago on osx, @stanzabird also changed it in win iirc. I think we should remove it completely.

• it makes RFP weaker for both the person using it and the rest of the RFP users, as it creates subsets with different values. we shouldn't undermine a built-in solution.
• we decided to keep it locked, meaning we weren't sold on it in the first place. I remember even @ohfp questioned whether it was a good idea.
• even in the faq we basically label it as an anti-feature.

RFP is all or nothing unfortunately, I think it's better to suggest users to live with the downsides than it is to make it worse. users still have the option to disable it if they feel like it doesn't fit their model, or override the theme with extension (fingerprintable, but not part of the browser and discouraged by us in the faq).

this change would require also updates of the settings repo and the website, I can take care of everything as I already have an idea of what needs changes.