Documentation/sphinx: drop modindex, we don't have python modules

The modindex is for python modules.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 71a276f..1139a7e 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -18,6 +18,4 @@
 ==================
 
 * :ref:`genindex`
-* :ref:`modindex`
 * :ref:`search`
-