python2.7-future: #22590 remove platform restriction (#22837)

Per #22590, `haskellPackages.servant` and by extension any package which transitively depends on `pythonX.Y-future` will fail to build on darwin even though there's apparently no reason why it can't be built there.

pkgs/top-level/python-packages.nix

@@ -11912,7 +11912,6 @@ in {
       downloadPage = https://github.com/PythonCharmers/python-future/releases;
       license = licenses.mit;
       maintainers = with maintainers; [ prikhi ];
-      platforms = platforms.linux;
     };
   };