Remove warning about exporting default constructor for `TokenMap`.
It's no longer necessary to worry about exporting the default constructor for `TokenMap`. The original concern was that exporting the default constructor would allow callers to break the invariant that there are no `mempty` values in the map. However, this is no longer possible, as the `MonoidMap` type handles this invariant automatically. Therefore, it's no longer necessary to have a warning. Nevertheless, this commit doesn't export the default constructor, as there currently isn't any need to do so.
