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/Classes/PeerConnection/RTCConfiguration.mm

@@ -404,8 +404,6 @@
 
 + (webrtc::SdpSemantics)nativeSdpSemanticsForSdpSemantics:(RTCSdpSemantics)sdpSemantics {
   switch (sdpSemantics) {
-    case RTCSdpSemanticsDefault:
-      return webrtc::SdpSemantics::kDefault;
     case RTCSdpSemanticsPlanB:
       return webrtc::SdpSemantics::kPlanB;
     case RTCSdpSemanticsUnifiedPlan:
@@ -415,8 +413,6 @@
 
 + (RTCSdpSemantics)sdpSemanticsForNativeSdpSemantics:(webrtc::SdpSemantics)sdpSemantics {
   switch (sdpSemantics) {
-    case webrtc::SdpSemantics::kDefault:
-      return RTCSdpSemanticsDefault;
     case webrtc::SdpSemantics::kPlanB:
       return RTCSdpSemanticsPlanB;
     case webrtc::SdpSemantics::kUnifiedPlan:
@@ -426,8 +422,6 @@
 
 + (NSString *)stringForSdpSemantics:(RTCSdpSemantics)sdpSemantics {
   switch (sdpSemantics) {
-    case RTCSdpSemanticsDefault:
-      return @"DEFAULT";
     case RTCSdpSemanticsPlanB:
       return @"PLAN_B";
     case RTCSdpSemanticsUnifiedPlan: