rtc: remove default aspect ratio constraint

Some devies may not be able to satisfy it and gUM will fail. Tested with a
crappy old camera which can only do VGA resolution at 4/3 aspect ratio.

modules/RTC/RTCUtils.js

  */
 const DEFAULT_CONSTRAINTS = {
     video: {
-        aspectRatio: 16 / 9,
         height: {
             ideal: 720,
             max: 720,