Remove Flipper references and gem

Summary

After Migrate Flipper Feature Flags to Unleash (#4039) completion, all CustomersDot Feature Flags should be using Unleash.

Flipper Feature Flags should no longer be created and all the Flipper related logic + setup can be removed.

Acceptance Criteria

• Remove all Flipper related gems from the Gemfile: flipper, flipper-active_record and flipper-active_support_cache_store (double check for if there is something else).
• Remove flipper related logic from the codebase: Feature::FlipperFeature + specs
• Drop the tables generated by the flipper gem to work with feature flags: features, feature_gates