Revert "BUG: only call RemoveProcessObject once from the process object"

This reverts commit db501388da066f51fcfc0a056ea362be231c475f.

Change-Id: If390245c5a7f50cb7dd92adbe56808822fc8fa15
Merge topic 'SIMPLEITK-453_AddEventCallbacks' into next

24c61352 Revert "BUG: only call RemoveProcessObject once from the process object"