Remove additional_panel_types and use_prometheus_endpoint feature flags

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

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](https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html