When building releasenotes (with 'tox -e releasenotes') the job fails
with the following error:
WARNING: Invalid configuration value found: 'language = None'. Update
your configuration to a valid language code. Falling back to 'en'
(English).
This caused publish-openstack-releasenotes-python3 post release job
failure as well. By removing the explicit set of language of None makes
the sphinx warning disappear.
Change-Id: I2482824f78901e14781bde49674bde4a9fd431ec
04e621b5bd