Remove references to va_online_scheduling_status_improvement feature flag

[JunTaoLuo]

Summary

• Removing obsolete feature flag

Related issue(s)

• Part of Remove any references of va_online_scheduling_status_improvement feature toggle va.gov-team#55085

Testing done

• Ensured existing tests pass

Acceptance criteria

• 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 (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

[LindseySaari]

@JunTaoLuo Reminder: Please open a support request so that we can manually remove the removed feature toggle in live envs. They are not auto removed from the DB and need to be manually removed according to the documentation (see 3a here)

[JunTaoLuo]

@LindseySaari We have an issue tracking the list of feature flags we need to remove in the production environment here. Since we still need to remove several more feature flags, we'll file the support request for all of them at once to hopefully make it simpler to deal with. FYI I have been removing the feature flags in the non-production environments (dev, staging, sandbox) so we only need support for removing the flags in the production environment.