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/d2coding/default.nix

@@ -1,10 +1,10 @@
-{ stdenv, fetchzip, unzip }:
+{ lib, fetchzip, unzip }:
 
 let
   version = "1.3.2";
   pname = "d2codingfont";
 
-in fetchzip rec {
+in fetchzip {
   name = "${pname}-${version}";
   url = "https://github.com/naver/${pname}/releases/download/VER${version}/D2Coding-Ver${version}-20180524.zip";
 
@@ -15,7 +15,7 @@ in fetchzip rec {
 
   sha256 = "1812r82530wzfki7k9cm35fy6k2lvis7j6w0w8svc784949m1wwj";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     description = "Monospace font with support for Korean and latin characters";
     longDescription = ''
       D2Coding is a monospace font developed by a Korean IT Company called Naver.