haskell: remove all but the latest LTS package sets (version 6.7)

This is the first step towards dropping Stackage support. We keep LTS 6.x
around because I don't want to downgrade our default compiler to GHC 7.x,
but once LTS 7.x comes out we'll switch our main package set to that and
drop Nightly.

More details are at:

  http://permalink.gmane.org/gmane.linux.distributions.nixos/20505

Closes https://github.com/NixOS/nixpkgs/issues/14897.

Also relevant:

 - https://github.com/NixOS/nixpkgs/issues/16130
 - https://github.com/commercialhaskell/stack/issues/2259