Remove leftover API helper for removed CI API

What does this MR do?

This removes an unused API helper for the already removed CI V1 API.

Why was this MR needed?

The API was removed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13652 but missed removing this helper

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13652

/cc @zj