Remove old column from users table

What does this MR do and why?

With this MR we're cleaning old column from the users table.

This is follow-up from !169769 (merged)

We're following https://docs.gitlab.com/development/database/avoiding_downtime_in_migrations/#dropping-columns documentation, and this is second step.

References

• Add pipl_users table and switch traffic to it. (!169769 - merged)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Checkout the branch
2. Run rails db:migrate
3. Verify that column has been removed! 🎉

MR acceptance checklist

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