Remove va_online_scheduling_GA4_migration feature toggle

[JunTaoLuo]

Summary

• Removing va_online_scheduling_GA4_migration feature toggle which is now obsolete.

Related issue(s)

• Remove any reference of va_online_scheduling_GA4_migration feature toggle va.gov-team#82973

Testing done

• Ensure existing test suite passes

Acceptance criteria

• Remove any references of the feature toggle va_online_scheduling_GA4_migration
• Changes have been updated in vets-api
• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

[ryan-mcneil]

I ❤️ Deletions.