haskell-cpython: revert "set to null"

This reverts commit 6a0b1b13b6. Please don't null
out the entire package just because its build is broken at the moment. If you
want to prevent users from compiling cpython, then haskell.lib.markBroken (or,
even better: markBrokenVersion) are the way to do it.

pkgs/development/haskell-modules/configuration-common.nix

@@ -893,9 +893,6 @@ self: super: {
   # https://github.com/aisamanra/config-ini/issues/12
   config-ini = dontCheck super.config-ini;
 
-  # We've remove cpython 3.4 from nixpkgs
-  cpython = null;
-
   # doctest >=0.9 && <0.12
   genvalidity-property = doJailbreak super.genvalidity-property;
   path = dontCheck super.path;