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