Do hero tokens get removed from the board when the old one card that brought them goes away? Like the witch hunters card. When it goes away, we can't tell if the heroes go away, or if they stay and just they no longer remove corruption tokens.

I believe the heros stay until some other card comes and removes them. It would weaken Slaanesh if they were removed once the card is discarded as many of its cards and motives are tied to nobles/heroes.

They stay until some other effect moves or removes them.