Delete forwarding endpoint

Deletes a forwarding endpoint identified by ref.slug. Delete any pipelines that target the endpoint first: if the endpoint is still referenced, the request is rejected with 412 (reason TELEMETRYRELAY_ENDPOINT_IN_USE) and the response lists the pipelines that must be deleted first. Returns 404 (reason TELEMETRYRELAY_ENDPOINT_NOT_FOUND) if no endpoint with that slug exists.