[18.8 Backport] Fix statement timeouts on p_ci_job_artifacts during pipeline deletion

What does this MR do and why?

Backport of !225515 (merged) (original fix) and !227837 (merged) (feature flag removal) to 18-8-stable-ee.

This fixes statement timeouts when deleting pipelines with many builds/job artifacts by using two-level batching (builds → artifacts) in Ci::Pipelines::DestroyAssociationsService instead of the problematic has_many :through JOIN query.

Cherry-picks both the original fix and the feature flag cleanup so that two-level batching is the default and only code path (no feature flag check).

Note: There was a minor conflict in app/models/ci/pipeline.rb around the private keyword placement, which was resolved cleanly.

References

• Feature issue: #582836 (closed)
• Rollout issue: #593102 (closed)
• Original fix MR: !225515 (merged)
• FF cleanup MR: !227837 (merged)

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