Experiment cleanup promote_premium_billing_page

What does this MR do and why?

The experiment is currently rolled out to 100% of users and has been deemed a success. The changes need to become an official part of the product.

• Experiment key / feature flag name: promote_premium_billing_page
• Epic or issue link: https://gitlab.com/groups/gitlab-org/-/epics/7173
• Rollout issue https://gitlab.com/gitlab-org/gitlab/-/issues/346275

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 #387305 (closed)