Remove obosolete SessionId setter/getter from PortAllocator.

webrtc/p2p/base/dtlstransportchannel.h

Index: webrtc/p2p/base/dtlstransportchannel.h
diff --git a/webrtc/p2p/base/dtlstransportchannel.h b/webrtc/p2p/base/dtlstransportchannel.h
index 7b8247a44903a5da6ac3535845f6f83a8c9678be..9cba249aca3d8c0b9201b41782738adbe0c8d785 100644
--- a/webrtc/p2p/base/dtlstransportchannel.h
+++ b/webrtc/p2p/base/dtlstransportchannel.h
@@ -118,7 +118,6 @@ class DtlsTransportChannelWrapper : public TransportChannelImpl {
   bool GetStats(ConnectionInfos* infos) override {
     return channel_->GetStats(infos);
   }
-  const std::string SessionId() const override { return channel_->SessionId(); }
 
   virtual bool SetSslMaxProtocolVersion(rtc::SSLProtocolVersion version);