Remove columns from the `application_settings` table

What does this MR do and why?

This MR drops some columns from the application_settings table.

Per https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html, step 2, we need to remove the columns from the database.

Based on:

• Deprecate column `static_objects_external_stora... (#348307 - closed)
• Allow setting the shard/replica separately for ... (#296882 - closed)
• Read any repo file (incl. private ones) via pos... (#374952 - closed)
• Remove repository_storages field (#429675)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.