Resolve "Extract EE specific files/lines for some controllers"

What does this MR do?

• 717a8eab 2018-05-16 | Extract EE-specific code from Projects::Settings::IntegrationsController [Rémy Coutable]
• 7343d9e8 2018-05-16 | Extract EE-specific code from Projects::Prometheus::MetricsController [Rémy Coutable]
• 93fc51ef 2018-05-16 | [EE] Introduce a new Keys::DestroyService service [Rémy Coutable]

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

I don't think so. CE backport is at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18994.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6020 (closed)