Revert "Merge pull request #27209 from nc6/nc/haskell-multiple-out"

This reverts commit dfb0f25484, reversing
changes made to 7f8ff02437. These changes broke
the ghcWithPackages wrapper:

    nix-shell -p "haskellPackages.ghcWithPackages (ps: [ps.mtl])" --run "ghc-pkg list mtl"
    /nix/store/szz84j5k1dy3jdashis6ws28d8l8zxxb-ghc-8.0.2-with-packages/lib/ghc-8.0.2/package.conf.d
        (no packages)

pkgs/development/haskell-modules/make-package-set.nix

@@ -44,7 +44,6 @@ let
       isLibrary = false;
       doHaddock = false;
       hyperlinkSource = false;      # Avoid depending on hscolour for this build.
-      enableSeparateEtcOutput = false; # The flag to support this is missing in old versions of cabal.
       postFixup = "rm -rf $out/lib $out/share $out/nix-support";
     });
     cpphs = overrideCabal (self.cpphs.overrideScope (self: super: {