Enable xindy for PDF build

This commit enables xindy for the PDF build. We can do this since the
bug[0] was already fixed.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1643664

Change-Id: I98aa27ea412fdc2d562fe845dd834550d6751a2b
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 7ce431e..69d6598 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -203,6 +203,3 @@
     ('index', 'doc-tempest.tex', u'Tempest Testing Project',
      u'OpenStack Foundation', 'manual'),
 ]
-
-# Disable usage of xindy https://bugzilla.redhat.com/show_bug.cgi?id=1643664
-latex_use_xindy = False