Drop obsolete "haskell-ng" and "haskellngPackages" aliases.

Haskell NG became the default implementation in Nixpkgs over half a year ago in
commit c0c82ea2. This means the "ng" names are
obsolete and should no longer be used. They'll continue to function in the
release-15.09 branch for the sake of backwards compatibility, but the master
branch no longer supports them.