Begin process for removing the onboarding_step_url column
What does this MR do and why?
Begin process for removing the onboarding_step_url column
- remove use as background migration for use has finished per: !145041 (comment 1801378958)
- begin https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns process
MR acceptance checklist
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Related to #443818 (closed)