Don't use field when saving related data

This should address my previous comment, https://github.com/tomchristie/django-rest-framework/pull/750#discussion_r3859448.

The serializer doing the saving won't have access to the fields for a deeply nested model, so the previous version was only suited for singly nested relations.  Now that we are using the `RelationsList` object it makes this a little easier to determine what we should be doing.