Drop unused preload from PipelineSerializer

What does this MR do?

Related to #321788

While working on #321788 I noticed that PipelineSerializer was preloading latest_statuses_ordered_by_stage that I don't believe is ever used. That scope is only called in a single place: legacy_stages_using_composite_status. AFAICT legacy stages are not rendered through Grape entities (only server-rendered ERBs, not JSON), plus we are in the process of removing them completely e.g. in !56987 (merged)

There is further evidence in this being unused in that pipeline tests are still passing while the number of queries issued is reduced (see spec diff).

Screenshots (strongly suggested)

Review app: https://gitlab-review-321788-dro-hve3ia.gitlab-review.app/

(since this was merely a pre-load it wouldn't change any behavior, just query count.)

Does this MR meet the acceptance criteria?

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