Remove namespace_storage_forks_cost_factor Feature Flag

What does this MR do and why?

Remove the namespace_storage_forks_cost_factor feature flag.

Removing the flag allows the storage cost factor for forks to be set in saas environments without adjusting the feature flag.

This is a gitlab.com-only feature that has been fully enabled for about 2 weeks now: https://gitlab.com/gitlab-org/gitlab/-/issues/419181#note_1512631600

See the epic for more context about the feature: https://gitlab.com/groups/gitlab-org/-/epics/10100

Feature flag cleanup issue: https://gitlab.com/gitlab-org/gitlab/-/issues/419194

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