Don't list M2M choices in OPTIONS requests. Refs #3751. (#4160)

rest_framework/metadata.py

@@ -138,7 +138,7 @@ class SimpleMetadata(BaseMetadata):
             field_info['children'] = self.get_serializer_info(field)
 
         if (not field_info.get('read_only') and
-            not isinstance(field, serializers.RelatedField) and
+            not isinstance(field, (serializers.RelatedField, serializers.ManyRelatedField)) and
                 hasattr(field, 'choices')):
             field_info['choices'] = [
                 {