Remove disableDeleteForTesting flag

Instrumenting the actual code to make tests work is generally a bad
practice that can let real issues slip into production code.

Removing this flag "Just Works" after fixing one ReplicationFanoutIT
test with buggy replicationDelay settings (the parent change of this
one).

Change-Id: Ia93192eeff0fb76c5c100de597a017b8b1f86025