Remove feature flags, use_prometheus_endpoint, additional_panel_types

What does this MR do?

This removes two feature flags

• environment_metrics_use_prometheus_endpoint
• environment_metrics_additional_panel_types

This also removes a lot of unused mocked data since we moved to the new prometheus endpoint, decoupling components from previous store states allowing for easier testing

#62094 (moved) #63845 (moved)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides