Remove Ci::ApplicationController

What does this MR do?

This MR proposes to remove app/controllers/ci/application_controller.rb, which defines Ci::ApplicationController class.

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

Now we are ready to remove Ci::ApplicationController, which can be merged into the base class ApplicationController, after !1987 (merged) and its subsequent MRs.

Why was this MR needed?

Just keep code clean.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests
  • [n/a] 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

What are the relevant issue numbers?

Closes #23068 (closed)