dwarf-fortress: use fetchFromGitHub directly

This avoids creating a second derivation to copy things from the
original. The new themes are now fixed output.

pkgs/games/dwarf-fortress/themes/default.nix

@@ -4,18 +4,12 @@ with builtins;
 
 listToAttrs (map (v: {
   inherit (v) name;
-  value = stdenv.mkDerivation {
+  value = fetchFromGitHub {
     name = "${v.name}-${v.version}";
-    src = fetchFromGitHub {
     owner = "DFgraphics";
     repo = v.name;
     rev = v.version;
     sha256 = v.sha256;
-    };
-    installPhase = ''
-      mkdir -p $out
-      cp -r data raw $out
-    '';
     meta = with lib; {
       platforms = platforms.all;
       maintainers = [ maintainers.matthewbauer ];