Migrate feature removal to RTK

[kyle-ssg]

Replace removeFlag with RTK Query mutation

Description

Replace the legacy removeFlag flow (AppActions → Dispatcher → feature-list-store controller) with the existing RTK Query
removeProjectFlag mutation from common/services/useProjectFlag.ts.

Current state

Feature flag deletion uses two parallel paths:

• RTK (new): FeaturesPage.tsx already uses useRemoveFeatureWithToast, which wraps useRemoveProjectFlagMutation. The
  removeProjectFlag mutation endpoint already exists in useProjectFlag.ts.
• Flux (old): CompareFeatures.js, CompareEnvironments.js, and WidgetPage.tsx still go through FeatureListProvider.removeFlag →
  AppActions.removeFlag → Dispatcher → feature-list-store.removeFlag controller, which manually calls the API, filters the store model,
  and triggers a 'removed' event.

Objectives

• Add removeProjectFlag store dispatch helper to common/services/useProjectFlag.ts (follows existing
  createProjectFlag/updateProjectFlag pattern) for use in class components
• Remove removeFlag controller method and REMOVE_FLAG case from common/stores/feature-list-store.ts
• Remove removeFlag action from common/dispatcher/app-actions.js
• Remove REMOVE_FLAG constant from common/dispatcher/action-constants.js
• Remove removeFlag method, render prop exposure, and 'removed' event listener from common/providers/FeatureListProvider.js
• Remove removeFlag from FeatureListProviderActions type in common/types/responses.ts
• Update web/components/CompareFeatures.js to use removeProjectFlag helper
• Update web/components/CompareEnvironments.js to use removeProjectFlag helper
• Update web/components/pages/WidgetPage.tsx to remove unused removeFlag destructuring and prop passing