Remove unused json encoder customizations

SQLAlchemyJSON adds the ability to encode SQLAlchemy models to Flask's jsonify. It auto-converts model instances to a dict with all model attributes as items. Until I recently stumbled over this, I never realized we had this functionality. I'm also pretty sure that we don't use it or rely on it in any way. I don't think we should use it.

@nd This was added in the very first commit made by you. Can you remember why you added it? Do you see a reason to keep it?