Removing core-spec.pdf from repository?

[Phil Ruffwind]

> I also dislike generated files in repos, but would like to point out
> that there are a few pages out there that reference the link
> https://github.com/ghc/ghc/blob/master/docs/core-spec/core-spec.pdf
> directly; these would brake. But there is probably nothing we can
> easily do about that.

Could replace it with a plain text file that says: "MOVED to
https://XXX"
The build system could then be tweaked to output the .pdf somewhere else
so it would not overwrite this dummy file.