db: drop __mapper_args__ that cause SQLAlchemy deprecation warnings

We don't really need them. There were only few uses left, and we seem to be
using .order_by() in all places where we really care about the ordering.

Gets rid of

<string>:2
<string>:2: SADeprecationWarning: Mapper.order_by is deprecated.Use Query.order_by() in order to affect the ordering of ORM result sets.