Add methods clean_on_xxxxxx() and clean_callbacks() to clean the existing callbacks in OverSIP::SIP::Proxy and OverSIP::SIP::Uac.
New design of `OverSIP::Modules::OutboundMangling` module: `add_outbound_to_contact()` now requires passing an `OverSIP::SIP::Proxy` as argument rather than a request, and it internally adds the callback to the 2XX response (for reverting the custom ;ov-ob param) so `remove_outbound_from_contact()` is no longer required and has been removed.