Remove unused onboarding_progresses columns

What does this MR do and why?

Related to #461074 (closed)

We believe that these columns that are ignored here in the onboarding_progresses table are not being used today. We also checked with data team that the use on their side is also deprecated: https://gitlab.com/gitlab-org/gitlab/-/issues/467305#note_2002328605.

So plan is to drop these columns according to https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html. This MR is the first step.

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