Revert "pythonPackages.setuptools: make compatible with hasPythonModule"

The packages with this change can not be evaluated with `nix-env --drv-path` or
`hydra-eval-jobs`: https://github.com/NixOS/nixpkgs/commit/859dc02fa3f42bca9b7770eb96f4fd8224cf1576

This reverts commit 859dc02fa3.

pkgs/development/python-modules/setuptools/default.nix

@@ -10,7 +10,6 @@ stdenv.mkDerivation rec {
   pname = "setuptools";
   version = "38.2.3";
   name = "${python.libPrefix}-${pname}-${version}";
-  pythonModule = python;
 
   src = fetchPypi {
     inherit pname version;