Removing WEBRTC_BUILD_LIBEVENT macro definition

This seems to be unused in WebRTC and a search in cs.chromium.orm shows
that it is never used in chromium too.

Interesting CL about this macro: https://codereview.webrtc.org/2153563003

BUG=None

Review-Url: https://codereview.webrtc.org/3006243002
Cr-Original-Commit-Position: refs/heads/master@{#19747}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 63f968e0cef08a88b711e121d61baa86f64fa0b5

rtc_base/BUILD.gn

diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 9572f17..902024f 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -289,10 +289,6 @@
   }
 }
 
-config("enable_libevent_config") {
-  defines = [ "WEBRTC_BUILD_LIBEVENT" ]
-}
-
 rtc_source_set("rtc_task_queue") {
   deps = [
     ":rtc_base_approved",
@@ -346,7 +342,6 @@
         "task_queue_posix.cc",
         "task_queue_posix.h",
       ]
-      all_dependent_configs = [ ":enable_libevent_config" ]
     } else {
       if (is_mac || is_ios) {
         sources = [