Remove old code related to scan_result_policy_read

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Why are we doing this work

Once Use database read model for merge request appr... (&9971 - closed) is complete and use_approval_policy_rules_for_approval_rules feature flag is default enabled, we want to remove the classes and methods that use/modify scan_result_policy_reads as it'll be replaced by approval_policy_rules.

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

These classes has to be removed:

• Security::SecurityOrchestrationPolicies::ProcessScanResultPolicyService
• Security::ProcessScanResultPolicyWorker
• Security::SecurityOrchestrationPolicies::SyncOpenedMergeRequestsService
• Security::Policy#delete_scan_result_policy_reads_for_project introduced by !176183 (merged)

⚠️ Since we will be removing a worker, we have to follow the guidelines for removing a worker as per docs

Verification steps