Delete unused sigslot SignalAddressReady and MSG_ID_ADDRESS_BOUND

Followup to https://webrtc-review.googlesource.com/c/src/+/207181

Bug: webrtc:11567
Change-Id: I604232eda0d5df7d9fe070926a37a4496924c637
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208280
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33317}

rtc_base/virtual_socket_server.h

@@ -369,9 +369,6 @@ class VirtualSocket : public AsyncSocket,
   // Removes stale packets from the network. Returns current size.
   size_t PurgeNetworkPackets(int64_t cur_time);
 
-  // For testing purpose only. Fired when client socket is bound to an address.
-  sigslot::signal2<VirtualSocket*, const SocketAddress&> SignalAddressReady;
-
  private:
   struct NetworkEntry {
     size_t size;