Remove Rake test_ci task

What does this MR do?

Removes test_ci from Rake tasks.

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

No. I confirmed there has been no test_ci usage in all history of .gitlab-ci.yml.

Why was this MR needed?

test_ci Rake task was originally introduced in 99a7bf88 by @dzaporozhetshas but has not been maintained for years (e.g., !5001 (merged)) and was replaced by gitlab:test Rake task, which is mentioned in doc/development/rake_tasks.md.

test_ci Rake task has not been integrated to GitLab CI jobs or described in any documentations for developers.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • [n/a] Has been reviewed by UX
  • [n/a] Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #44902 (closed)