docs: simplify conf.py and fix compatibility with upcoming Sphinx 6.0

Change-Id: Ib68b2cf40c9b3e74251bb87aa76ca18b7e371f3c
diff --git a/docs/named_data_theme/layout.html b/docs/named_data_theme/layout.html
index 973ff89..e6488e1 100644
--- a/docs/named_data_theme/layout.html
+++ b/docs/named_data_theme/layout.html
@@ -40,9 +40,8 @@
 
           <h3>{{ _('Developer documentation') }}</h3>
           <ul>
-            <li class="toctree-l1"><a class="reference external" href="https://redmine.named-data.net/projects/nlsr/wiki">NLSR Wiki</a></li>
             <li class="toctree-l1"><a class="reference internal" href="doxygen/annotated.html">API documentation (doxygen)</a></li>
-            <li class="toctree-l1"><a class="reference internal" href="code-style.html">ndn-cxx Code Style and Coding Guidelines</a></li>
+            <li class="toctree-l1"><a class="reference external" href="https://redmine.named-data.net/projects/nlsr/wiki">NLSR Wiki</a></li>
           </ul>
           {%- endblock %}