Revert "Leverage dispatch_queue_create_with_target when possible."

This reverts commit de86381161651816c078adeb354902b15d03a35b.

Reason for revert: Fails downstream project, """fatal error: 'rtc_base/system/gcd_helpers.h' file not found"""

Original change's description:
> Leverage dispatch_queue_create_with_target when possible.
> 
> Replacing dispatch_queue_create followed by
> dispatch_set_target_queue with dispatch_queue_create_with_target
> is claimed to be source of GCD performance improvement:
> https://developer.apple.com/videos/play/wwdc2017/706/
> Video since 40 min. Slides since 199.
> 
> Bug: webrtc:9055
> Change-Id: I0136f7faaef0951a7ad243bc8772f3ee952d5470
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168491
> Reviewed-by: Tommi <tommi@webrtc.org>
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Commit-Queue: Yura Yaroshevich <yura.yaroshevich@gmail.com>
> Cr-Commit-Position: refs/heads/master@{#30781}

TBR=tommi@webrtc.org,kthelgason@webrtc.org,yura.yaroshevich@gmail.com

Change-Id: I47fafa47afa2c825c8f100253d8a1f035203d9e8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9055
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170361
Reviewed-by: Alex Loiko <aleloi@google.com>
Commit-Queue: Alex Loiko <aleloi@google.com>
Cr-Commit-Position: refs/heads/master@{#30785}

5 files changed, 4 insertions, 64 deletions

diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 2e4138e458..5cb3fea8da 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -487,7 +487,6 @@ if (is_mac || is_ios) {
       ":checks",
       ":logging",
       "../api/task_queue",
-      "system:gcd_helpers",
       "//third_party/abseil-cpp/absl/strings",
     ]
   }
diff --git a/rtc_base/system/BUILD.gn b/rtc_base/system/BUILD.gn
index 61e7e678a6..937fec11e2 100644
--- a/rtc_base/system/BUILD.gn
+++ b/rtc_base/system/BUILD.gn
@@ -60,13 +60,6 @@ if (is_mac || is_ios) {
     deps = [ "..:checks" ]
     libs = [ "Foundation.framework" ]
   }
-
-  rtc_library("gcd_helpers") {
-    sources = [
-      "gcd_helpers.h",
-      "gcd_helpers.m",
-    ]
-  }
 }
 
 rtc_source_set("thread_registry") {
diff --git a/rtc_base/system/gcd_helpers.h b/rtc_base/system/gcd_helpers.h
deleted file mode 100644
index a8df0a9d83..0000000000
--- a/rtc_base/system/gcd_helpers.h
+++ /dev/null
@@ -1,29 +0,0 @@
-/*
- *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#ifndef RTC_BASE_SYSTEM_GCD_HELPERS_H_
-#define RTC_BASE_SYSTEM_GCD_HELPERS_H_
-
-#include <dispatch/dispatch.h>
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-DISPATCH_RETURNS_RETAINED DISPATCH_WARN_RESULT DISPATCH_NOTHROW dispatch_queue_t
-RTCDispatchQueueCreateWithTarget(const char* label,
-                                 dispatch_queue_attr_t attr,
-                                 dispatch_queue_t target);
-
-#ifdef __cplusplus
-}
-#endif
-
-#endif  // RTC_BASE_SYSTEM_GCD_HELPERS_H_
diff --git a/rtc_base/system/gcd_helpers.m b/rtc_base/system/gcd_helpers.m
deleted file mode 100644
index ff113266a1..0000000000
--- a/rtc_base/system/gcd_helpers.m
+++ /dev/null
@@ -1,22 +0,0 @@
-/*
- *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#include "rtc_base/system/gcd_helpers.h"
-
-dispatch_queue_t RTCDispatchQueueCreateWithTarget(const char *label,
-                                                  dispatch_queue_attr_t attr,
-                                                  dispatch_queue_t target) {
-  if (@available(iOS 10, macOS 10.12, tvOS 10, watchOS 3, *)) {
-    return dispatch_queue_create_with_target(label, attr, target);
-  }
-  dispatch_queue_t queue = dispatch_queue_create(label, attr);
-  dispatch_set_target_queue(queue, target);
-  return queue;
-}
\ No newline at end of file
diff --git a/rtc_base/task_queue_gcd.cc b/rtc_base/task_queue_gcd.cc
index 2276f635c5..cb516cc4cc 100644
--- a/rtc_base/task_queue_gcd.cc
+++ b/rtc_base/task_queue_gcd.cc
@@ -24,7 +24,6 @@
 #include "api/task_queue/task_queue_base.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
-#include "rtc_base/system/gcd_helpers.h"
 
 namespace webrtc {
 namespace {
@@ -68,16 +67,16 @@ class TaskQueueGcd : public TaskQueueBase {
 };
 
 TaskQueueGcd::TaskQueueGcd(absl::string_view queue_name, int gcd_priority)
-    : queue_(RTCDispatchQueueCreateWithTarget(
-          std::string(queue_name).c_str(),
-          DISPATCH_QUEUE_SERIAL,
-          dispatch_get_global_queue(gcd_priority, 0))),
+    : queue_(dispatch_queue_create(std::string(queue_name).c_str(),
+                                   DISPATCH_QUEUE_SERIAL)),
       is_active_(true) {
   RTC_CHECK(queue_);
   dispatch_set_context(queue_, this);
   // Assign a finalizer that will delete the queue when the last reference
   // is released. This may run after the TaskQueue::Delete.
   dispatch_set_finalizer_f(queue_, &DeleteQueue);
+
+  dispatch_set_target_queue(queue_, dispatch_get_global_queue(gcd_priority, 0));
 }
 
 TaskQueueGcd::~TaskQueueGcd() = default;