Duplicate optional label

Optional fields have this form: `Name optional (optional)`. Based on my understanding, the first one is added by this library and the second one is added by gcanti/tcomb-form-native.
@alvaromb is it safe to remove [this statement](https://github.com/APSL/tcomb-form-native-json-schema/blob/master/lib/options.js#L65)? LMK if I should open a PR.