Remove deprecated //rtc_base:async_resolver.

Bug: webrtc:9987
Change-Id: I8146786abc94c9cdc820495d7e86282ad2250144
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/202241
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33017}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index f6efa8e..7af8e89 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -667,12 +667,6 @@
   }
 }
 
-rtc_source_set("async_resolver") {
-  # TODO(bugs.webrtc.org/9987): Remove when downstream clients have
-  # been redirected on //rtc_base:threading.
-  sources = []
-}
-
 rtc_library("net_helpers") {
   sources = [
     "net_helpers.cc",