giflib: Remove xml dependencies

These should have been in nativeBuildInputs anyways.
But it seems the docbook generation isn't needed when downloading the tarball

It didn't change the output anyways.
So if there are any docs that were supposed to be built that needs to be fixed somehow.

pkgs/development/libraries/giflib/default.nix

@@ -31,8 +31,6 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = lib.optionals stdenv.isDarwin [ fixDarwinDylibNames ];
 
-  buildInputs = [ xmlto docbook_xml_dtd_412 docbook_xsl libxml2 ];
-
   passthru.tests.static = pkgsStatic.giflib;
 
   meta = {