Nix Flake: Remove hlint

`hlint` should not be necessary. However, this is an opinion up for debate:

We do not really need the `hlint` executable because it is integrated in HLS.

However, `hlint` is used by a GitHub Action, which we may want to replicate locally.

Also, the `fmt.sh` script is downloading an (unknown) `hlint` version which will
not work on NixOS anyways. In my opinion, if `fmt.sh` is still used (I can not
find any usages) we should fix the script so it is reproducible.

Author: dschrempf

flake.nix

@@ -63,8 +63,6 @@
             (gen-hls-changelogs pkgs.haskellPackages)
             # For the documentation
             pythonWithPackages
-            # @guibou: I'm not sure this is needed.
-            hlint
             (pkgs.haskell.lib.justStaticExecutables (pkgs.haskell.lib.dontCheck pkgs.haskellPackages.opentelemetry-extra))
             capstone
             stylish-haskell