Remove is_free_namespace property from helpers

What does this MR do and why?

It removes is_free_namespace from group and namespace helpers as it's no longer needed by the frontend.

Note: this MR depends on !107014 (merged) being merged first.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Disable namespace_storage_limit feature flag (bin/rails console then Feature.disable(:namespace_storage_limit)).
2. Enable automatic_purchased_storage_allocation setting (bin/rails console then ::Gitlab::CurrentSettings.update(automatic_purchased_storage_allocation: true))
3. Navigate to an owned free or paid namespace
4. From left sidebar > Settings > Usage Quotas > Storage
5. Observe the storage top section with the cards view

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.

Related to #375651