Remove 'modelLoadedCallback' (that was specific for Lab needs)

This is a breaking change. To adapt old code talking to Lab, which
probably looks like:

phone = new iframePhone.ParentEndpoint("#myiframe", afterConnectedCallback, modelLoadedCallback);

you should replace it with:

phone = new iframePhone.ParentEndpoint("#myiframe", afterConnectedCallback);
phone.addListener("modelLoaded", modelLoadedCallback);