Delete cascade for 1-n-DBRelationships
Example use-case: MFAMethod.user in uffd is a 1-n-DBRelationship with backref User.mfa_methods. Calling user.mfa_methods.clear()
sets MFAMethod.user to NULL for all matching objects, but does not delete the objects.
If SQLAlchemy can auto-delete rows when a column is set to NULL, that would be fine too.