data/fonts: cleanup

* make font derivations fixed-output where applicable
 * fix dead links
 * `stdenv.lib` -> `lib` where `stdenv` is not involved
 * remove `meta.platforms = [ unix ]` and `meta.platforms = [ linux ]` because the restriction has no sense for data packages

pkgs/data/fonts/poly/default.nix

@@ -46,6 +46,5 @@ stdenv.mkDerivation rec {
     homepage = http://www.fontsquirrel.com/fonts/poly;
     license = stdenv.lib.licenses.ofl;
     maintainers = with stdenv.lib.maintainers; [ relrod ];
-    platforms = with stdenv.lib.platforms; linux;
   };
 }