Remove link to modindex

The documentation build does not generate any module index, thus remove
the link to the page. The page
http://docs.openstack.org/infra/zuul/py-modindex.html
does not exist.

Change-Id: Iafa9388a2b1186afb43314ea2d427c75da1e8c9f
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 3c793da..98f8992 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -29,6 +29,5 @@
 ==================
 
 * :ref:`genindex`
-* :ref:`modindex`
 * :ref:`search`