Remove `dora_projects_comparison_subgroups` feature flag

What does this MR do and why?

Remove dora_projects_comparison_subgroups feature flag now that the solution has been validated. The feature has been enabled globally since March 14th.

References

Screenshots or screen recordings

How to set up and validate locally

• Mock DORA metrics for a project within a subgroup

[
  [1.month.ago, 5, 1, 5, 2],
  [2.month.ago, 10, 3, 3, 3]
].each do |date, deploys, lead_time_for_changes, time_to_restore_service, incidents_count|
  Dora::DailyMetrics.create!(
    deployment_frequency: deploys,
    lead_time_for_changes_in_seconds: lead_time_for_changes * 1.day.to_i,
    time_to_restore_service_in_seconds: time_to_restore_service * 1.day.to_i,
    incidents_count: incidents_count,
    environment: Project.find(PROJECT_ID).default_environment,
    date: date
  )
end

• Navigate to the Value streams dashboard for the parent group (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards/value_streams_dashboard)
• Find the Projects by DORA metric panel and ensure the subgroup project appears in the list of projects

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #524641 (closed)