Remove various redundant indexes

## What does this MR do?

This MR removes various redundant and unused indexes. See c3d5c536d9c77dbe183744cb19f2cdab7ee606fd for more information.

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

No.

## Why was this MR needed?

Unused indexes are a waste of storage space and slow down database writes. If they are ever needed again we can re-create them. There's no point in hoarding indexes because you _may_ need them in the future.

## What are the relevant issue numbers?

#20767

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5759