docs: remove trailing header

This header has been there since the document was added, but contains
nothing. So let's get rid of it.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3443>

docs/dispatch.html

@@ -268,8 +268,6 @@ dispatch stub.</p>
 <code>src/mesa/glapi/glapi.c</code> just before <code>glprocs.h</code> is
 included.</p>
 
-<h2 id="autogen">4. Automatic Generation of Dispatch Stubs</h2>
-
 </div>
 </body>
 </html>