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

The modindex is for python modules.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

Documentation/index.rst

@@ -18,6 +18,4 @@ Indices and tables
 ==================
 
 * :ref:`genindex`
-* :ref:`modindex`
 * :ref:`search`
-