Remove `hls-stan-plugin`

[michaelpj]

It doesn't work on any recent GHC, and is blocked by stan itself being updated, but stan is pretty dead. I think we should remove it.

cc @uhbif19

[uhbif19]

@michaelpj

Yes, I can see your reasoning.

Seems like there is a branch working with new GHC. kowainik/stan#423 (comment)

I can ask if it gonna be merged and if not create fork. Will it work for you (if Stan gonna work with new GHCs?)?

[michaelpj]

Sure: if we can get the plugin working again then there's no reason to drop it. It's only if it looks like it's dead that we might as well drop it. And stan does look fairly dead.

[michaelpj]

Done