Replace query used by `OrphanedRemediationsCleanupWorker`

What does this MR do and why?

This MR is a spin-off from Deleted `vulnerability_remediations` leave behi... (!160579 - merged) • Michael Becker • 17.4

based on this thread

While trying to fix the issue of orphaned uploads, we ran into the issue of the existing query timing out in production.

That resulted in two spin-off MRs:

• This MR: removing an unnecessary join from the existing query
• !161891 (merged): adding additional metadata to the worker's logs

With these two MRs merged, we will have better insight into how to further improve this query in production

SQL

Current

SELECT
    "vulnerability_remediations"."id"
FROM
    "vulnerability_remediations"
    LEFT OUTER JOIN "vulnerability_findings_remediations" ON "vulnerability_findings_remediations"."vulnerability_remediation_id" = "vulnerability_remediations"."id"
    LEFT OUTER JOIN "vulnerability_occurrences" ON "vulnerability_occurrences"."id" = "vulnerability_findings_remediations"."vulnerability_occurrence_id"
WHERE
    "vulnerability_occurrences"."id" IS NULL

• EXPLAIN: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/30572/commands/94811

New

SELECT
    "vulnerability_remediations"."id"
FROM
    "vulnerability_remediations"
    LEFT OUTER JOIN "vulnerability_findings_remediations" ON "vulnerability_findings_remediations"."vulnerability_remediation_id" = "vulnerability_remediations"."id"
WHERE
    "vulnerability_findings_remediations"."id" IS NULL

• EXPLAIN: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/30572/commands/94812

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Related to #462724 (closed)