Resolve "Remove routes that we do not use anymore"

Originally opened at !4059 (closed) by @strzibny.

---

What does this MR do?

I tried to clean up the routes as mentioned in #14584 (closed) 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 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together