Contact sales experiment cleanup

What does this MR do and why?

The experiment is currently rolled out to 100% of users and has been deemed a success as we've seen an increase in the trial to paid conversion rate. The changes need to become an official part of the product.

• Experiment results:
• Experiment key / feature flag name: group_contact_sales
• Epic or issue link: https://gitlab.com/gitlab-org/gitlab/-/issues/348345

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 #362247