Revert "clean up List db queries"

This reverts commit 41f27a4a66.

I forgot that update() can only be done on a query result, not on an object, so we will need to go back to querying in order to update rather than saving.

bookwyrm/models/list.py

@@ -101,10 +101,15 @@ class List(OrderedCollectionMixin, BookWyrmModel):
     def remove_from_group(cls, owner, user):
         """remove a list from a group"""
 
-        cls.objects.filter(group__user=owner, user=user).all().update(
+        memberships = GroupMember.objects.filter(group__user=owner, user=user).all()
+
+        for membership in memberships:
+            # remove this user's group-curated lists from the group
+            cls.objects.filter(group=membership.group, user=membership.user).update(
                 group=None, curation="closed"
             )
 
+
 class ListItem(CollectionItemMixin, BookWyrmModel):
     """ok"""