Fix build with gtk-doc-1.32

This bug was reported here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=939956

The issue is that gtk-doc-1.32 no longer generates empty tree_index
files.  The recommendation is that liblangtag be modified to no longer
reference what used to be an empty tree_index, but is now a missing
file.  The same bug report contained the patch implemented here, which
removes the reference to a now-missing tree_index file.

The fix should already be provided in newer versions of liblangtag.

diff --git a/docs/liblangtag-docs.sgml b/docs/liblangtag-docs.sgml
index 1234567..1234567 100644
--- a/docs/liblangtag-docs.sgml
+++ b/docs/liblangtag-docs.sgml
@@ -58,10 +58,6 @@
     </section>
 
   </chapter>
-  <chapter id="object-tree">
-    <title>Object Hierarchy</title>
-     <xi:include href="xml/tree_index.sgml"/>
-  </chapter>
   <index id="api-index-full">
     <title>API Index</title>
     <xi:include href="xml/api-index-full.xml"><xi:fallback /></xi:include>