Delete SignalQueueDestroyed

It was used only to break the circular dependency between SocketServer
and Thread at destruction time. Replaced with a method call to
SetMessageQueue(nullptr).

Bug: webrtc:11943
Change-Id: I0606d473ad79655cca28411bb02c21e21d2d7220
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215587
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33807}

rtc_base/virtual_socket_server.h

@@ -33,7 +33,7 @@ class SocketAddressPair;
 // interface can create as many addresses as you want.  All of the sockets
 // created by this network will be able to communicate with one another, unless
 // they are bound to addresses from incompatible families.
-class VirtualSocketServer : public SocketServer, public sigslot::has_slots<> {
+class VirtualSocketServer : public SocketServer {
  public:
   VirtualSocketServer();
   // This constructor needs to be used if the test uses a fake clock and
@@ -259,11 +259,6 @@ class VirtualSocketServer : public SocketServer, public sigslot::has_slots<> {
                                             uint32_t samples);
   static double Evaluate(const Function* f, double x);
 
-  // Null out our message queue if it goes away. Necessary in the case where
-  // our lifetime is greater than that of the thread we are using, since we
-  // try to send Close messages for all connected sockets when we shutdown.
-  void OnMessageQueueDestroyed() { msg_queue_ = nullptr; }
-
   // Determine if two sockets should be able to communicate.
   // We don't (currently) specify an address family for sockets; instead,
   // the currently bound address is used to infer the address family.