Get rid of node->wildcard_child pointer

Some info is in issue #216 (closed). It should be quite straightforward.

The change also uncovered one other problem in the code: node's 'removed NSEC' flag could remain in the node after applying and signing an UPDATE. I forced clearing of this flag in the adjusting function. This probably renders the clearing in zone signing (zone-sign.c:569) redundant, but I'm not 100% sure about that. But feel free to remove it from zone signing if it seems alright and tests pass.