Revert "Fix venv creation in Python environments"

This commit reverts all python changes from 234bb31f61.

pkgs/development/interpreters/python/cpython/2.7/default.nix

@@ -318,6 +318,8 @@ in with passthru; stdenv.mkDerivation ({
     inherit passthru;
 
     postFixup = ''
+      # Include a sitecustomize.py file. Note it causes an error when it's in postInstall with 2.7.
+      cp ${../../sitecustomize.py} $out/${sitePackages}/sitecustomize.py
     '' + lib.optionalString strip2to3 ''
       rm -R $out/bin/2to3 $out/lib/python*/lib2to3
     '' + lib.optionalString stripConfig ''