docs: Remove unused GObject hierarchy chapter
We don't export GObject types, only boxed GTypes.
Closes
<xi:include href="xml/graphene-gobject.xml"/>
</chapter>
- <chapter id="object-tree">
- <title>Object Hierarchy</title>
- <xi:include href="xml/tree_index.sgml"/>
- </chapter>
<index id="api-index">
<title>API Index</title>
<xi:include href="xml/api-index-full.xml"><xi:fallback /></xi:include>
