Remove obsolete sentence from docs

Author: peterthomassen

docs/api-guide/serializers.md

@@ -845,8 +845,6 @@ Here's an example of how you might choose to implement multiple updates:
         class Meta:
             list_serializer_class = BookListSerializer
 
-It is possible that a third party package may be included alongside the 3.1 release that provides some automatic support for multiple update operations, similar to the `allow_add_remove` behavior that was present in REST framework 2.
-
 #### Customizing ListSerializer initialization
 
 When a serializer with `many=True` is instantiated, we need to determine which arguments and keyword arguments should be passed to the `.__init__()` method for both the child `Serializer` class, and for the parent `ListSerializer` class.