Remove Marginalia feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

We've had Marginalia enabled in production for over 10 months now: gitlab-org/database-team/team-tasks#30 (closed)

We're unlikely to turn it off at this point as it's becoming more important for our database performance analysis, as in issues like gitlab-com/gl-infra/scalability#566 (moved).

Closes #211472 (closed).