Fix ManyRelatedField.choices

Use choices from self.child_relation. 
We don't need to repeat ourself because can get out of sync.

rest_framework/relations.py

@@ -388,16 +388,4 @@ class ManyRelatedField(Field):
 
     @property
     def choices(self):
-        queryset = self.child_relation.queryset
-        iterable = queryset.all() if (hasattr(queryset, 'all')) else queryset
-        items_and_representations = [
-            (item, self.child_relation.to_representation(item))
-            for item in iterable
-        ]
-        return OrderedDict([
-            (
-                six.text_type(item_representation),
-                six.text_type(item) + ' - ' + six.text_type(item_representation)
-            )
-            for item, item_representation in items_and_representations
-        ])
+        return self.child_relation.choices