Revert "Add Throwable.UNASSIGNED_STACK to well known fields"

This reverts commit d80668611e1982d50374da03377542a7863ba8e5.

Reason for revert: Appears to break Serialization of some Throwable subclasses.

Bug: 35910877
Bug: 109930347
Change-Id: Ic78e63229f946175c629ae76b82a277ed5f12e0f
3 files changed