babel extractors are now registered via python entry points,
so there is no need to declare babel extractors in babel configs.
This change is important to make translation work in Django 2.2.
django-babel does not work with Django 2.2 and looks unmaintained
for over two years. The horizon team is thinking to switch the extractor
to enmerkar (a fork of django-babel) to make extraction of translation
string work again near future. It is important to drop the extractor
definition to make the transition smooth.
Change-Id: I68b0eecd6b17be0e059fccdbced5784151c0e492
Akihiro Motoki