doc/reference: don't add empty object hierarchy chapter

The object hierarchy section is empty because there are no GObjects in the libtasn1 API. With gtk-doc 1.30 onwards if there are no objects then the object hierarchy file won't exist (see here), resulting in a failure when building the documentation:

../libtasn1-docs.xml:39: element include: XInclude error : could not load ../xml/tree_index.sgml, and no fallback was found