Drop syncrhonized from JobUpdateEventSubscriber

This fixes a startup deadlock.

Testing Done:
./gradlew -Pq build

Manually verified that all delegated calls to the JobUpdateController
are already protected by the storage write-lock.

Rather than add a potentially-flaky regression test (like
the one added in https://reviews.apache.org/r/25556/)
I'd prefer to prioritize adding runtime deadlock detection
(https://issues.apache.org/jira/browse/AURORA-800).

Bugs closed: AURORA-801

Reviewed at https://reviews.apache.org/r/26422/