Remove any reference of va_online_scheduling_GA4_migration feature toggle

[jenniemc]

Description

Since the analytic team will be implementing the GA4 migration, we no longer need to reference the feature toggle va_online_scheduling_GA4_migration in the our codebase.

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Acceptance Criteria

• Remove any references of the feature toggle va_online_scheduling_GA4_migration
• Delete the feature toggle va_online_scheduling_GA4_migration in staging and production
• Changes have been updated in vets-website and vets-api
• Documentation is updated

Definition of Done

• All acceptance criteria are met
• Documentation is updated

[JunTaoLuo]

Code for the feature flag removal has been merged for both vets-website and vets-api and both are deployed to production. The feature flags have also been removed from dev/sandbox/staging environments. Removal in the production environment is pending in #84111