Remove 'sphinxcontrib.autohttp.flask' from sphinx config
Remove 'sphinxcontrib.autohttp.flask' from sphinx config

* After updating Sphinx to 1.6.2 Sphinx this extension got broken
  and our Mistral "doc" CI job too
* We don't use Flask in the project anyway

Change-Id: I051ec26e155344684b69bad6a50f2162febbc928

openstack/mistral,StackStorm/mistral,openstack/mistral,StackStorm/mistral