Resolve "[CI Minutes] [Feature flag] Remove 'new_route_ci_minutes_purchase' feature flag"

What does this MR do and why?

Removes new_route_ci_minutes_purchase feature flag as the feature is deemed stable on production.

Screenshots or screen recordings

Feature	Screenshot
Buy additional minutes takes us to the right link
Loads the GitLab checkout flow in the same window
Purchase from personal namespace points to CDot flow as expected

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #337064 (closed)