Follow up: Clean up redundant tests in audit worker

In MR !118513 (merged), with addition of rspec files for individual destination strategy spec files such as ee/spec/lib/audit_events/strategies/instance_external_destination_strategy_spec.rb, there are several redundant tests in ee/spec/workers/audit_events/audit_event_streaming_worker_spec.rb which needs to be cleanedup. This is a followup issue for the same.