glib,docs: Remove "object-tree" chapter
glib,docs: Remove "object-tree" chapter

gtk-doc-1.30+ does *not* create tree_index.sgml in case the library does
not provide GObjects, which is the case of LibvirtGLib. In previous
versions, though, an empty file would be generated.

This seems to be a regression which hit *a* *lot* of different projects,
all of them having to take the very same approach.

Signed-off-by: Fabiano Fidêncio <2931bae082ef9bc04523ba759c03a4778316d48f@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

libvirt/libvirt-glib,libvirt/libvirt-glib,libvirt/libvirt-glib