Remove stale feature flag: environment_metrics

We need to clean up feature flag usage in the code.
Our goal is to delete references of the "environment_metrics" feature.
We need to delete the feature flag check so that the code path is no longer guarded by the feature flag.
These are the occurrences of this feature flag in this repository:
- [`frontend/web/components/pages/features/components/FeatureMetricsSection.tsx#L15`](https://github.com/Flagsmith/flagsmith/blob/5dd0918f08d7151037df0db4b5a925aca0a1c9b8/frontend/web/components/pages/features/components/FeatureMetricsSection.tsx#L15)