Remove obosolete SessionId setter/getter from PortAllocator.

webrtc/p2p/base/p2ptransportchannel.cc

Index: webrtc/p2p/base/p2ptransportchannel.cc
diff --git a/webrtc/p2p/base/p2ptransportchannel.cc b/webrtc/p2p/base/p2ptransportchannel.cc
index ec1dfc733f80ed3275038b17608f9cd6531aff9f..085185b172fffe7191a4117ff28fe809c29ca7cb 100644
--- a/webrtc/p2p/base/p2ptransportchannel.cc
+++ b/webrtc/p2p/base/p2ptransportchannel.cc
@@ -447,7 +447,7 @@ void P2PTransportChannel::MaybeStartGathering() {
       }
     } else {
       AddAllocatorSession(allocator_->CreateSession(
-          SessionId(), transport_name(), component(), ice_ufrag_, ice_pwd_));
+          transport_name(), component(), ice_ufrag_, ice_pwd_));
       allocator_sessions_.back()->StartGettingPorts();
     }
   }