Clean-up registration_verification feature flag

What does this MR do and why?

This issue is to cleanup the registration_verification feature flag. This feature flag and associated experiment were never enabled in production.

How to set up and validate locally

• Go through sign up process. Make sure it isn't broken.

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 #420661 (closed)