remove gudev and gtk-doc

The library moved to:
  https://git.gnome.org/browse/libgudev/

Author: kaysievers

systemd/docs/layout.html

@@ -4,8 +4,6 @@
   <a href="../man/systemd.index.html">Index </a>·
   <a href="../man/systemd.directives.html">Directives </a>·
   <a href="index.html">Python </a>·
-  <a href="../libudev/index.html">libudev </a>·
-  <a href="../libudev/index.html">gudev </a>
   <span style="float:right">systemd {{release}}</span>
   <hr />
 {% endblock %}