Follow the new PTI for document build [Updated]

This review already follows the new PTI
https://review.openstack.org/#/c/529006/

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

Change-Id: I2f4638c71a625df7efa49818f8e40d69ae750301

setup.cfg

@@ -85,11 +85,6 @@ oslo_middleware =
 cinder.database.migration_backend =
     sqlalchemy = oslo_db.sqlalchemy.migration
 
-[build_sphinx]
-all_files = 1
-warning-is-error = 1
-build-dir = doc/build
-source-dir = doc/source
 
 [egg_info]
 tag_build =