Remove obsolete, broken, and unnecessary versions of GHC.

These versions have been removed:

 - 6.4.2-binary.nix
 - 6.4.2.nix
 - 6.6.1.nix
 - 6.8.2.nix
 - 6.8.3.nix
 - 6.10.1-binary.nix
 - 6.10.1.nix
 - 6.10.2.nix
 - 6.10.3.nix
 - 6.11.nix
 - 6.12.1-binary.nix
 - 6.12.1.nix
 - 6.12.2.nix
 - 7.0.1.nix
 - 7.0.2.nix
 - 7.0.3.nix
 - 7.2.1.nix
 - 7.4.1.nix
 - 7.6.1.nix
 - 7.6.2.nix
 - 7.8.3-binary.nix

As a rule of thumb, we keep the latest version in every major release. If
someone feels up to the task of fixing versions 6.4.x, 6.6.x, and 6.8.x, then
please don't hesitate to revive those builds.

Fixes https://github.com/NixOS/nixpkgs/issues/5630.