Remove metrics_dashboard_visibility_switching_available feature flag

What does this MR do?

This removes the metrics_dashboard_visibility_switching_available feature flag, as the feature flag was created as a safety in case the backend portion didn't make it for %12.10

issue: #201924 (closed)

Backend MR: !29371 (merged)

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides