Do not use the second MemoryManager in Serializer

* Now that we handle MemoryManager::getMoreMemory(), it is no more needed
  (memory wise) to use the other MM and it actually might be unwise to do so
  (it could allocate an extra heap which might not be needed if there is no GC).
* Also, keeping this would make any sharing of the secondMM more complicated.
* The Serializer does not depend anymore on such low-level memory details.