Drop reference to webrtc_overrides/.../task_queue.h

Since cl
https://chromium-review.googlesource.com/c/chromium/src/+/664807,
chromium no longer uses it. We need to stop referring to it from
webrtc, before it can be deleted in chromium.

BUG=webrtc:8166

Review-Url: https://codereview.webrtc.org/3015513002
Cr-Commit-Position: refs/heads/master@{#19901}

rtc_base/BUILD.gn

@@ -332,15 +332,9 @@ rtc_source_set("rtc_task_queue") {
 # provided implementation should be used. An external implementation should
 # depend on rtc_task_queue_api.
 rtc_source_set("rtc_task_queue_api") {
-  if (build_with_chromium) {
-    sources = [
-      "../../webrtc_overrides/rtc_base/task_queue.h",
-    ]
-  } else {
-    sources = [
-      "task_queue.h",
-    ]
-  }
+  sources = [
+    "task_queue.h",
+  ]
   deps = [
     ":rtc_base_approved",
   ]