model [nfc]: Remove TODO(server-6) where there's nothing to do

This isn't a case of special backward-compat code to be trimmed out;
the backwards compatibility was simply that older servers wouldn't
exercise this code.

Fixes-partly: zulip#1837

Author: chrisbobbe

lib/api/model/model.dart

@@ -97,7 +97,7 @@ enum CustomProfileFieldType {
   link(apiValue: 5),
   user(apiValue: 6),
   externalAccount(apiValue: 7),
-  pronouns(apiValue: 8), // TODO(server-6) newly added
+  pronouns(apiValue: 8),
   unknown(apiValue: null);
 
   const CustomProfileFieldType({