Revert rP24167 / Keep track of serialized objects using a GCHashTable
This reverts rP24167. That diff had two issues:
- It modifies the JS objects, which means subsequent serialization in quicksave are 'dirty'.
- It doesn't work with non-extensible objects. That's rather annoying, and has already caused problems.
It also revert rP24466, which was necessary because of the second issue.
Differential Revision: https://code.wildfiregames.com/D3336