Remove obosolete SessionId setter/getter from PortAllocator.

webrtc/p2p/base/transportchannel.h

Index: webrtc/p2p/base/transportchannel.h
diff --git a/webrtc/p2p/base/transportchannel.h b/webrtc/p2p/base/transportchannel.h
index 411591c8496192391c1de80f1e013ddc0007416e..9b0ac7bd5bcae60bd95d6c1c5c264d363c2e1160 100644
--- a/webrtc/p2p/base/transportchannel.h
+++ b/webrtc/p2p/base/transportchannel.h
@@ -66,10 +66,6 @@ class TransportChannel : public sigslot::has_slots<> {
     return TransportChannelState::STATE_CONNECTING;
   }
 
-  // TODO(mallinath) - Remove this API, as it's no longer useful.
-  // Returns the session id of this channel.
-  virtual const std::string SessionId() const { return std::string(); }
-
   const std::string& transport_name() const { return transport_name_; }
   int component() const { return component_; }