Removes UserDetails.registration_objective column

What does this MR do and why?

This MR removes the registration_objective column from the UserDetails table as part of the cleanup process following the migration to the new onboarding_status JSONB column.

Background:

• The registration_objective column data was successfully migrated to the onboarding_status JSONB column in GitLab 17.9
• The backfill migration BackfillOnboardingStatusRegistrationObjective completed 4 months ago (Issue #498740)
• Application code was updated to use the new JSONB column in the cutover MR
• The column has been ignored in the model since the cutover

Timeline:

• 17.9: Backfill migration introduced
• 17.11: Required stop version (ensures BBM completion for self-managed)
• 18.1-18.2: Column ignored in model, BBM finalized
• 18.3: Safe to remove column (this MR)

References

• Cutover MR
• Issue for migrating registration_objective into the onboarding_status jsonb column

Screenshots or screen recordings

No UI changes.

Before	After

How to set up and validate locally

Run migration + verify column has been removed.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #532965 (closed)