RTCIceRole redundancy removal nit

[robin-raymond]

We had in the spec the RTCIceRole being specified twice, once in constructor and once in start method. It's truly optional (conflict can resolve) although recommended to set so not sure why we need it mandatory in constructor too.

[Constructor(optional RTCIceListener iceListener), 
     Constructor(RTCIceOptions options)]
interface RTCIceTransport {
    [...]
    void                      start (RTCIceParameters remoteParameters, optional RTCIceRole role);
    void                      stop ();
    [...]
};