Contacts who have unsubscribed from a mailing list can't resubscribe

When someone unsubscribes from a mailing list, they're still in the group but the status is marked "Removed". So if that person wants to resubscribe, it sees that they're still in the group and gives an error message that looks like this (for a mailing list group called Action Alert): "You are already subscribed in Action Alert, your subscription is Removed." There's no other way to resubscribe without having an admin do it for them.

This patch fixes that problem.