Revert "docbook-xsl: Install dbtoepub"

This reverts commit 4e32f5bda3.

IMHO, it's not desirable to make docbook-xsl (and by extension a
gazillion packages that depend on it) pull in Ruby. (For example, I
just noticed that wget depends on ruby now...)

pkgs/data/sgml+xml/stylesheets/xslt/docbook-xsl/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, ruby }:
+{ lib, stdenv, fetchurl }:
 
 let
 
@@ -10,8 +10,6 @@ let
       inherit sha256;
     };
 
-    buildInputs = [ ruby ];
-
     dontBuild = true;
 
     installPhase = ''
@@ -23,9 +21,6 @@ let
       # Backwards compatibility. Will remove eventually.
       mkdir -p $out/xml/xsl
       ln -s $dst $out/xml/xsl/docbook
-
-      ln -sv $dst/epub/bin $out
-      chmod +x $out/bin/dbtoepub
     '';
 
     meta = {