In Queens cycle, the code of django_openstack_auth was merged into
the horizon repository. The master branch of django_openstack_auth
will be retired. (horizon blueprint merge-openstack-auth)
This commit drops django_openstack_auth related code from DevStack.
_prepare_message_catalog_compilation in lib/horizon was used only
in install_django_openstack_auth, so it is dropped too.
Change-Id: If9467c520a1e07d1968b29e485df0097330356bc