Experiment classes cleanup

Summary

Some experiments were cleaned up, but their experiment files were not.

Plan

• Remove any stale(no feature flag defined) experiments from /app/experiments, right now those are:
  • app/experiments/logged_out_marketing_header_experiment.rb
  • app/experiments/force_company_trial_experiment.rb
  • app/experiments/empty_repo_upload_experiment.rb
• Create a test to make sure any future cleanups take remove the class files - perhaps a danger check created like danger/feature_flag/Dangerfile for this might work.