Follow the new PTI for document build

This review already follows the new PTI
https://review.openstack.org/#/c/530875/

However, we also should remove [build_sphinx]
section as described in:
http://lists.openstack.org/pipermail/openstack-dev/2018-March/128594.html

Change-Id: Ifbfe80024731d2c552a5aab90e025f9ebd593c4d

setup.cfg

@@ -47,12 +47,5 @@ tempest.cm =
 oslo.config.opts =
     tempest.config = tempest.config:list_opts
 
-[build_sphinx]
-all-files = 1
-# warning can be generated by using GENERATE_TEMPEST_PLUGIN_LIST='False'
-warning-is-error = 0
-build-dir = doc/build
-source-dir = doc/source
-
 [wheel]
 universal = 1