Don Bluth didn't work on any of the sequels. It was MGM animation not Sullivan Bluth studios who did the sequels and the series. I don't think they're moles anyway, I always thought of them as just colouring on his fur. Either way I'd say the reason that it's gone is because they artists didn't pay enough attention to Charlies colouring and what not from the first movie or they just reckon he looked better without them.