Fix evaluation of `cabal2nix`

`cabal2nix` fails to evaluate due to attempting to
evaluate `pkgs.haskellPackages.hackage-db_2_1_0`, which
does not exist.  However, the default
`pkgs.haskellPackages.hackage-db` is already version 2.1.0,
so the fix is simple: go back to using the default version.