Drop old unused non-partitioned duo_workflows_checkpoints table

What does this MR do and why?

Drop old unused non-partitioned duo_workflows_checkpoints table

No activerecord model refers to it.

References

• Related to Chore: Drop old non-partitioned duo_workflows_c... (#555538 - closed)
• Switch Ai::DuoWorkflows::Checkpoint model to us... (!196629 - merged)
• Partition the `duo_workflows_checkpoints` table... (#470469 - closed)
• https://docs.gitlab.com/development/migration_style_guide/#dropping-a-database-table
• https://docs.gitlab.com/development/database/database_dictionary/#dropping-tables

How to set up and validate locally

bin/rails db:migrate:up:main VERSION=20250901060725
bin/rails db:migrate:down:main VERSION=20250901060725

MR acceptance checklist

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