Remove root_storage_statistics_calculate_forks Feature Flag

What does this MR do and why?

Remove the root_storage_statistics_calculate_forks feature flag. With this flag removal, the Namespaces::RootStorageStatistics#recalculate! method calculates and stores the size of public, internal, and private forks of projects whenever it is invoked.

This flag is globally enabled on gitlab.com and has caused no issues. See the rollout issue here: https://gitlab.com/gitlab-org/gitlab/-/issues/408139

See the feature issue here: https://gitlab.com/gitlab-org/gitlab/-/issues/399368

Feature flag cleanup issue: https://gitlab.com/gitlab-org/gitlab/-/issues/409745

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.