Fix a delete-zone-playing-voice crash (#1447)

I actually had a 'TODO repack zones' in teh code, so replace that with
a call to the repack zone weak ref operator on delete.

Closes #1445

src/engine/group.h

@@ -166,8 +166,9 @@ struct Group : MoveableOnly<Group>,
 
         auto res = std::move(zones[idx]);
         zones.erase(zones.begin() + idx);
-        // REPACK WEAK REFS TODO
         res->parentGroup = nullptr;
+
+        postZoneTraversalRemoveHandler();
         return res;
     }