django: remove SchemaEditor.quote_value() TODO

fixes googleapis#227

Author: timgraham

django_spanner/schema.py

@@ -199,8 +199,7 @@ def add_index(self, model, index):
         super().add_index(model, index)
 
     def quote_value(self, value):
-        # TODO: a real implementation:
-        # https://github.com/orijtech/django-spanner/issues/227
+        # A more complete implementation isn't currently required.
         return str(value)
 
     def _alter_field(self, model, old_field, new_field, old_type, new_type,