Remove feature flag environment_metrics_use_prometheus_endpoint

What does this MR do?

Removes the environment_metrics_use_prometheus_endpoint feature flag from the code base.

closes #62094 (moved)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content