Resolve "Remove `events` from `notification_settings`"

What does this MR do?

Remove an unused column.

Are there points in the code the reviewer needs to double check?

I don't think so - I am waiting to confirm that this is safe, though: https://gitlab.com/gitlab-com/infrastructure/issues/2365

Why was this MR needed?

We don't need the column any more!

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • All builds are passing
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33620 (closed).