Delete an escalation policy

Deletes the policy — but only while nothing points at it. A policy some recipient or group still uses answers 409 with the list of who uses it, and stays. That refusal is deliberate: detaching would not break the setting, it would silently EMPTY its ladder — the account-wide floor is a single message with no timeout step, so an unanswered call would be repeated by nobody and handed to nobody. Repeating the call on an already-deleted policy returns 404.