remove legacy offerOptions.mandatory

[fippo]

also the webrtc bug indicated seems not to exist anymore

[fippo]

var pc = new RTCPeerConnection();
navigator.mediaDevices.getUserMedia({audio: true})
  .then(s => pc.addTrack(s.getTracks()[0], s))
  .then(() => pc.createOffer({offerToReceiveAudio: false}).then(offer => console.log(offer.sdp)));

generates a a=sendonly mediaSection