Take a SIP line back from a recipient

Unbinds the recipient and removes the mirrored internal_sip contact, leaving the line itself in place and free for somebody else. This is the reversible half of the pair: POST /api/v1/extensions with the same extension_uuid puts it back. To destroy the line instead, use DELETE on the line itself. Answers 422 when the line is not an operator line or holds nobody — there is nothing to take back.