Experiment cleanup discover security

Experiment Tracking: Display security navigation in left sidebar to a cohort of net new .com signups

gitlab-org/growth/team-tasks#54 (closed)

• Remove the experiment code & start ignoring the column !45636 (merged)
• Delete the feature flags from production and staging
• Drop the unused column !48221 (merged)
• Remove the ignore rule !49121 (merged)