all-packages.nix: remove the haskellPackages_ghcHEAD package set from the top-level

That package set is broken because GHC 7.10.1-rc1 no longer supports old-style
database files as used by the pkgs/build-support/cabal/default.nix builder.

A working packages set is in "haskell-ng.packages.ghcHEAD".