break: drop unused riscv64 enable option

[Xeonacid]

Fixes #2509

cc @mayeut

[mayeut]

This will break some users, holding for a future major or minor release.

[henryiii]

I think a future minor release should be fine, like 3.3, since it was experimental and probably doesn't have a huge number of users. Fine to wait for 4.0 if others think differently, though.

[mayeut]

Something like 3.3 sounds good

[mayeut]

We're now after 3.3.0, I think it's fine to merge, even if the next version is 3.3.1