Remove the send_user_confirmation_email column

What does this MR do and why?

What

Removes the old send_user_email_confirmation column from the application_settings table

Why

As per https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-the-column-release-m1, the old setting was ignored in 15.8 and can now be removed

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371638 (closed)