Delete AsyncSocket class, merge into Socket class

Bug: webrtc:13065
Change-Id: I13afee2386ea9c4de0e4fa95133f0c4d3ec826e8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/227031
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34787}

rtc_base/null_socket_server.cc

@@ -34,10 +34,4 @@ rtc::Socket* NullSocketServer::CreateSocket(int /* family */, int /* type */) {
   return nullptr;
 }
 
-rtc::AsyncSocket* NullSocketServer::CreateAsyncSocket(int /* family */,
-                                                      int /* type */) {
-  RTC_NOTREACHED();
-  return nullptr;
-}
-
 }  // namespace rtc