Remove SdpSemantics::kDefault

This adds confusion to the native API and is only needed for
Chromium UMA metrics, so the appropriate metrics have been moved
upstream and kDefault option removed.

Bug: chromium:811683
Change-Id: I666d7f7793765b8d6edcd99416c8b6c957766f00
Reviewed-on: https://webrtc-review.googlesource.com/59261
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22864}

sdk/objc/Framework/Headers/WebRTC/RTCConfiguration.h

@@ -65,7 +65,6 @@ typedef NS_ENUM(NSInteger, RTCEncryptionKeyType) {
 
 /** Represents the chosen SDP semantics for the RTCPeerConnection. */
 typedef NS_ENUM(NSInteger, RTCSdpSemantics) {
-  RTCSdpSemanticsDefault,
   RTCSdpSemanticsPlanB,
   RTCSdpSemanticsUnifiedPlan,
 };
@@ -155,14 +154,11 @@ RTC_EXPORT
  *  will also cause RTCPeerConnection to ignore all but the first a=ssrc lines
  *  that form a Plan B stream.
  *
- *  For users who only send at most one audio and one video track, this
- *  choice does not matter and should be left as Default.
- *
  *  For users who wish to send multiple audio/video streams and need to stay
- *  interoperable with legacy WebRTC implementations, specify PlanB.
+ *  interoperable with legacy WebRTC implementations or use legacy APIs,
+ *  specify PlanB.
  *
- *  For users who wish to send multiple audio/video streams and/or wish to
- *  use the new RTCRtpTransceiver API, specify UnifiedPlan.
+ *  For all other users, specify UnifiedPlan.
  */
 @property(nonatomic, assign) RTCSdpSemantics sdpSemantics;