Delete the document and restore the agent route the number had before.
Deliberately not gated on carrier eligibility or on a live key. This is the only way to
disarm a policy route, and the safe way to turn a route off must never depend on the
carrier account still verifying - the same rule the legacy clear already follows.
The route is only moved when the number is actually on policy: a number a later
inbound-voice-routing PATCH already returned to an agent keeps that agent instead of
having it silently cleared by this call.