chore(feature-flag): feature flag legacy code cleanup

[rafaeltonholo]

Contribution Summary

Linked Issue/Ticket: Closes #11337

RFC / Technical Design (if applicable): RFC 0004: Add a Declarative Feature Flag Catalog / Technical Design 0002: Declarative Feature Flag Catalog

Description

Remove all the legacy code from the old Feature Flag Architecture.

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Contribution Checklist

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• This contribution is in Kotlin where possible
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• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).

[wmontwe]

LGTM