Clean up partition_aware_pipeline_preload feature flag

What does this MR do and why?

The partition_aware_pipeline_preload feature flag reached 100% rollout on GitLab.com on 2026-08-26 and has been stable since. This removes the flag check from Ci::Partitionable::AssociationFinder::PipelineRelationPreload#preload_associations, so partition-aware bulk pipeline preloading is now always active, and deletes the flag's YAML definition and the associated "flag disabled" specs.

Related issues:

• Resolves #603166 (closed) (rollout issue)
• #601100 (closed) (parent tracking issue for per-model conversions)

Screenshots or screen recordings

Not applicable — internal cleanup, no user-facing change.

How to set up and validate locally

1. Checkout this branch.
2. Run the affected specs:

   bundle exec rspec spec/models/concerns/ci/partitionable/association_finder_spec.rb \
     ee/spec/models/merge_trains/car_spec.rb \
     ee/spec/models/sbom/occurrence_spec.rb \
     ee/spec/models/vulnerabilities/feedback_spec.rb

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.

After merge

• Run /chatops gitlab run feature delete partition_aware_pipeline_preload --dev --pre --staging --staging-ref --production
• Close #603166 (closed)