Remove CMS feature toggle: FEATURE_TELEPHONE_MIGRATION_V1

[jilladams]

BLOCKED BY FE UPDATES in #19968

Description

FEATURE_TELEPHONE_MIGRATION_V1 is a CMS feature toggle created to ship phone number interface updates.

We need to:

• Update the CMS to keep the presentation of new phone number fields, regardless of feature toggle state
• Remove the CMS feature toggle

Acceptance criteria

• FE has stopped using the feature toggle in content-build and changes are deployed
• CMS feature toggle is removed
• New phone number fields are visible on content forms for:
  • VAMC Facility: Mental health phone number
  • VAMC Billing & Insurance
  • Staff profile