Remove Nix expressions

Including Nix expressions in Haskell projects, particularly core infrastructure,
is harmful for the following reasons:

1. The expressions are never maintained. In this case, I should know since I'm
the one who isn't maintaining them.
2. In the unlikely event a Nix expression is up-to-date, it still only works
with a particular version of Nixpkgs. Unless Nixpkgs is included in a submodule,
this is of no use to anybody.
3. Creating Nix expressions for Haskell packages is automated anyway and anyone
who is interested in having them knows how to generate them.
4. We who are working on Cabal need to be "eating our own dog food" or "wearing
the hair shirt" as the case may be.

[ci skip]

Author: ttuegel