Remove `environment_metrics_use_prometheus_endpoint` feature flag

Summary

We want to remove the environment_metrics_use_prometheus_endpoint feature flag from the code base once the feature is enabled on GitLab.com - https://gitlab.com/gitlab-org/gitlab-ce/issues/58516.

Involved components

Changes should be fairly minimal, but span across both frontend and backend.

See:

• app/controllers/projects/environments_controller.rb