Revert "Prevent races with GC when transferring objects between threads"

This reverts commit 7585b91bfc77b8e4b821ccfa716fa86e46455275.

Reason for revert: Seems to be causing fatal error: 
  Checkpoint flag set without pending checkpoint

Change-Id: I98ea653e629d73e854907115583afed3ed5ac68e
Test: None.
Bug: 67838964
8 files changed