Remove transaction in ProcessScanResultPolicyWorker

What does this MR do and why?

Addresses Investigate/fix Security::ProcessScanResultPoli... (#408143 - closed)

This MR removes the usage of transaction in Security::ProcessScanResultPolicyWorker behind the feature flag remove_scan_result_policy_transaction

The reason we had to do everything within a transaction was due to race condition while concurrent execution of the worker for a same project and configuration_id. But now that we have idempotency and deduplication enabled as part of Make ProcessScanResultPolicyWorker idempotent a... (!116485 - merged), we should be good without a transaction.

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