Cleanup application settings spec

What does this MR do?

EE already includes specs for the ApplicationSettingsController, so ensure future changes will be easier to merge to EE.

Also check the EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1498

Why was this MR needed?

In 9-0-stable some conflicts are resolved, but not very clean. This will clean this up. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10070#note_25898584

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together