model [nfc]: Remove a TODO(server-6) on a still-nullable field

The doc for this says
> This field is only included when its value is `true`.
so the TODO(server-6) isn't reason enough to make it a required
field.

Fixes-partly: zulip#1837

Author: chrisbobbe

lib/api/model/model.dart

@@ -69,7 +69,7 @@ class CustomProfileField {
   final String name;
   final String hint;
   final String fieldData;
-  final bool? displayInProfileSummary; // TODO(server-6)
+  final bool? displayInProfileSummary;
 
   CustomProfileField({
     required this.id,