Remove hlintCheck

[srid]

treefmt can do this: https://github.com/srid/haskell-template/pull/83/files

And hlint does not need to be run in the devShell, which is all the more reason to decouple it from haskell-flake. haskell-flake should perhaps have a 'recommendations' section in the README, advising people to use treefmt-nix if they want goodies like auto-formatting, hlint, etc.