hard-host-depends: remove gtk-doc

This would only be needed if we were building board packages with the
"doc" use flag, which we don't. So it and many things can go away:

 app-text/docbook-dsssl-stylesheets
 app-text/docbook-sgml-dtd
 app-text/openjade
 app-text/opensp
 app-text/rarian
 app-text/scrollkeeper
 app-text/scrollkeeper-dtd
 app-text/yelp-tools
 dev-util/gtk-doc
 dev-util/itstool
 gnome-extra/yelp-xsl