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

What does this MR do?

This MR removes the feature flag used in the development of maintenance mode: #217895 (closed)

Which reveals the setting in Admin Area

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.