Resolve "Remove routes that we do not use anymore"

_Originally opened at !4059 by @strzibny._

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## What does this MR do?

> I tried to clean up the routes as mentioned in #14584
I used the following script and manually verified the changes: https://gist.github.com/strzibny/4ccbda7dcf67ef6719dcb047014e1ea7
After this patch, the script still gives me the following routes:
Unused routes (7): oauth/authorizations#show doorkeeper/tokens#create doorkeeper/tokens#revoke oauth/applications#edit oauth/applications#show doorkeeper/token_info#show omniauth_callbacks#(?-mix:(?!))
Here is a relevant code from doorkeeper on drawing these routes: https://github.com/doorkeeper-gem/doorkeeper/blob/5ea4575863781940fe6996692c6b41f2ad71d90e/lib/doorkeeper/rails/routes.rb#L25

## What are the relevant issue numbers?

Closes #14584.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] 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 !5499