deleting both a lookup list and the related fk_ft field raises an error on migrations

To reproduce:
Create a subrecord that related fk_or_ft field to a new lookup list.

create migrations.

delete the fk_or_ft field and the lookup list.

makemigrations/migrate will raise an error.

I assume this is due to this logic https://github.com/openhealthcare/opal/blob/v0.13.0/opal/core/fields.py#L95