Revert "[AC] Fix NPE. If the client sends multiple unregister requests at the...

Revert "[AC] Fix NPE. If the client sends multiple unregister requests at the same time, the client request could be null before the next request is processed."

This reverts commit 21b89197.

Reason for revert: Need to address WARNING reported by Error Prone: [SynchronizeOnNonFinalField] Synchronizing on non-final fields is not safe: if the field is ever updated, different threads may end up locking on different objects.

Change-Id: I97acc0c933795e90e9e7fa0e72798259acf862dc