Resolve "Experiment classes cleanup"

What does this MR do and why?

1. Remove class files of stale experiments (removed before) from /app/experiments/
2. Add a danger check to make sure future experiment cleanups also remove the class files if exisit.

Screenshots or screen recordings

How to set up and validate locally

1. Delete config/feature_flags/experiment/build_ios_app_guide_email.yml
2. Commit the change,
3. Run bin/rake danger_local

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 #410613 (closed)