Remove reference to va_online_scheduling_required_schedulable_param feature flag

[JunTaoLuo]

Summary

This PR removes the obsolete feature flag va_online_scheduling_required_schedulable_param and its references in this repo. This flag is currently enabled in staging and production.

For testing disable the following feature flags and confirm scheduling appointments are working as expected. Refer to this checklist for a full list of scenarios.

Related issue(s)

• Resolves Remove any references of va_online_scheduling_required_schedulebale_param feature toggle - (vets-api)  va.gov-team#63682.

Testing done

• New code is covered by existing unit tests
• Prior to this change, schedulable is forced to true only if the feature flag is enabled. Now the feature flag is removed, this parameter is always set to true.
• Regression tests for the current behavior is tested without setting the va_online_scheduling_required_schedulable_param flag to be true

Screenshots

N/A

What areas of the site does it impact?

Appointments (VAOS)

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 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

[ajmagdub]

LGTM