Remove docs for GitLab CI Service API

What does this MR do?

Documentation update: http://docs.gitlab.com/ce/api/services.html

Old GitLab CI Service API description will be removed from the docs.

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

n/a

Why was this MR needed?

Obsolete information is harmful than nothing.

What are the relevant issue numbers?

Closes #19167 (closed)

Does this MR meet the acceptance criteria?

• [n/a] CHANGELOG entry added
• Documentation created/updated
• [n/a] API support added
• Tests
  • Added for this feature/bug
  • 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