Remove reference to __v_ prefix

As of getodk/central-frontend#1024, the `UserPreferences` object creates a proxy around a shallow reactive object whereby the proxy gates access to the reactive object's properties. If something tries to read a property that isn't associated with a known user preference, the proxy throws an error. However, Vue seems to be trying to read properties that start with `__v_`. That seems to be something internal to Vue, not part of the Vue API. For now, the `UserPreferences` proxy explicitly allows `__v_` properties to be read. However, since `__v_` is undocumented, it'd be ideal if we could find a way to avoid this reference to it.