Remove already enabled `environment_metrics_additional_panel_types` feature flag from codebase

Task

Remove the :environment_metrics_additional_panel_types feature flag and act as if it has been set to true.

• Used in the frontend as environmentMetricsAdditionalPanelType.

This feature is already enabled in staging and .com:

/chatops run feature get environment_metrics_additional_panel_types --staging
/chatops run feature get environment_metrics_additional_panel_types

["Group:3925021", "Group:9970"] has no effect becuase the value of the flag is passed to all users at https://gitlab.com/gitlab-org/gitlab/blob/master/app/controllers/projects/environments_controller.rb#L17