Remove legacy empty task_queue BUILD targets and build arg

Now that they are unused

Bug: webrtc:10191
Change-Id: Id5f1b70ed8e084555ca809fef2a75359266f2ca1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/126980
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27070}

api/task_queue/BUILD.gn

@@ -28,11 +28,6 @@ rtc_source_set("task_queue") {
   ]
 }
 
-# TODO(danilchap): Remove this empty target when downstream project stop depending on it.
-rtc_source_set("task_queue_factory") {
-  visibility = [ "*" ]
-}
-
 rtc_source_set("task_queue_test") {
   visibility = [ "*" ]
   testonly = true

rtc_base/BUILD.gn

@@ -483,10 +483,6 @@ rtc_source_set("rtc_task_queue") {
   ]
 }
 
-rtc_source_set("rtc_task_queue_api") {
-  visibility = [ "*" ]
-}
-
 if (rtc_enable_libevent) {
   rtc_source_set("rtc_task_queue_libevent") {
     visibility = [ "../api/task_queue:default_task_queue_factory" ]

webrtc.gni

@@ -136,11 +136,6 @@ declare_args() {
   # See http://clang.llvm.org/docs/SanitizerCoverage.html .
   rtc_sanitize_coverage = ""
 
-  # Links a default implementation of task queues to targets
-  # that depend on the target rtc_task_queue. Set to false to
-  # use an external implementation.
-  rtc_link_task_queue_impl = true
-
   if (current_cpu == "arm" || current_cpu == "arm64") {
     rtc_prefer_fixed_point = true
   }
@@ -218,8 +213,6 @@ declare_args() {
   rtc_build_usrsctp = !build_with_mozilla
 
   # Enable libevent task queues on platforms that support it.
-  # rtc_link_task_queue_impl must be set to true for this to
-  # have an effect.
   if (is_win || is_mac || is_ios || is_nacl || is_fuchsia) {
     rtc_enable_libevent = false
     rtc_build_libevent = false