Remove ci_instance_variables_ui feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes the ci_instance_variables_ui feature flag

#299879 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content