Elm: fix build by ignoring the build instructions provided by upstream

According to <https://github.com/elm-lang/Elm/issues/384>, Elm's release
archive comes with a Setup.hs that cannot compile an Elm release. Duh!

Replacing the custom Setup.hs file with a dummy version fixes this
issue.

Close <https://github.com/NixOS/nixpkgs/issues/2089>.