Delete AsyncSocket temporary alias

The class was deleted in
https://webrtc-review.googlesource.com/c/src/+/227031.

Bug: webrtc:13065
Change-Id: Ica18110c3ac441fc7ab768e46a073f409601c1c0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/229301
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34847}

rtc_base/BUILD.gn

@@ -844,7 +844,6 @@ rtc_library("socket") {
     ":socket_address",
     "third_party/sigslot",
   ]
-  absl_deps = [ "//third_party/abseil-cpp/absl/base:core_headers" ]
   if (is_win) {
     deps += [ ":win32" ]
   }

rtc_base/socket.h

@@ -25,7 +25,6 @@
 #include "rtc_base/win32.h"
 #endif
 
-#include "absl/base/attributes.h"
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/socket_address.h"
 #include "rtc_base/third_party/sigslot/sigslot.h"
@@ -144,10 +143,6 @@ class Socket {
   RTC_DISALLOW_COPY_AND_ASSIGN(Socket);
 };
 
-// TODO(bugs.webrtc.org/13065): Old alias, delete ASAP, when downstream code is
-// updated.
-using AsyncSocket ABSL_DEPRECATED("bugs.webrtc.org/13065") = Socket;
-
 }  // namespace rtc
 
 #endif  // RTC_BASE_SOCKET_H_