Cleanup bigint conversion for shared_runners_duration

What does this MR do and why?

Cleanup bigint conversion for shared_runners_duration

This is the step to cleanup the bigint conversion for:

• ci_project_monthly_usages.shared_runners_duration
• ci_namespace_monthly_usages.shared_runners_duration

See https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#remove-the-trigger-and-old-integer-columns-release-n--2

• backfill is done at !129573 (merged)
• column swap is done in (16.5) at !132225 (merged)

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Related to #424825 (closed)