This field wasn't used for anything, and I think it has very limited
use for debugging, since fundamentally, it'll almost always have a
value within the hour of the actual timestamp in FillState, and any
more fine-grained logging we might want would be available in the
analytics job's own logs.
The proximal reason to remove it is that apparently Django's
model_to_dict doesn't support auto_now fields, and that caused some
trouble when working on adding more complete import/export support for
analytics data.
