Remove redundant spec under ci_variable

What does this MR do and why?

Describe in detail what your merge request does and why.

• Remove qa/qa/specs/features/browser_ui/4_verify/ci_variable/add_remove_ci_variable_spec.rb per gitlab-org/quality/quality-engineering/team-tasks#1647 (closed)
• This also involves removing unused related qa-selectors and page object methods.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.