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
profiles: enable python bindings in libxml2

Not required any more but helps existing SDKs upgrade.
Merge pull request #806 from marineam/python

hard-host-depends: remove gtk-doc