build: Drop DocBook schema validation

It was not actually used because we did not pass it the manual.

And if we fix it by replacing the Bash `$combined` variable by a Nix one,
it will fail because we only have RelaxNG schema for Docbook 5.0,
which unlike later versions is too limited (e.g. does not allow
emphasis element within filename element).

- https://tdg.docbook.org/tdg/5.0/filename.html
- https://tdg.docbook.org/tdg/5.2/filename.html

default.nix

@@ -60,11 +60,9 @@ in pkgs.stdenv.mkDerivation {
   name = "nix-pills";
 
   src = sources;
-  buildInputs = with pkgs; [ jing libxslt ];
+  buildInputs = with pkgs; [ libxslt ];
 
   installPhase = ''
-    jing ${pkgs.docbook5}/xml/rng/docbook/docbook.rng $combined
-
     # Generate the HTML manual.
     dst=$out/share/doc/nix-pills
     mkdir -p $dst