Revert "[scheduler] Registering the current task executor."

This reverts commit 0c8cf257.

Reason for revert: The upstream CL got reverted.

Original change's description:
> [scheduler] Registering the current task executor.
> 
> I'm saving to TLS the current task executor on registration, and providing
> it for the base::CurrentThread() trait.
> 
> Bug: 968047
> Change-Id: I753213ff57249ed55d10cb0a576ec56975a4088e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642562
> Reviewed-by: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Karolina Soltys <ksolt@chromium.org>
> Commit-Queue: Karolina Soltys <ksolt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#666291}

TBR=skyostil@chromium.org,alexclarke@chromium.org,ksolt@chromium.org

Change-Id: Iebd1add6a23939b48ae2c25335b49ce2869a2f97
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 968047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645777Reviewed-by: Karolina Soltys <ksolt@chromium.org>
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666303}

base/task/task_executor.cc

@@ -6,10 +6,8 @@
 
 #include <type_traits>
 
-#include "base/no_destructor.h"
 #include "base/task/task_traits.h"
 #include "base/task/task_traits_extension.h"
-#include "base/threading/thread_local.h"
 
 namespace base {
 
@@ -30,18 +28,12 @@ static_assert(
     TaskTraitsExtensionStorage::kInvalidExtensionId == 0,
     "TaskExecutorMap depends on 0 being an invalid TaskTraits extension ID");
 
-ThreadLocalPointer<TaskExecutor>* GetTLSForCurrentTaskExecutor() {
-  static NoDestructor<ThreadLocalPointer<TaskExecutor>> instance;
-  return instance.get();
-}
-
 }  // namespace
 
 void RegisterTaskExecutor(uint8_t extension_id, TaskExecutor* task_executor) {
   DCHECK_NE(extension_id, TaskTraitsExtensionStorage::kInvalidExtensionId);
   DCHECK_LE(extension_id, TaskTraitsExtensionStorage::kMaxExtensionId);
   DCHECK_EQ((*GetTaskExecutorMap())[extension_id - 1], nullptr);
-  GetTLSForCurrentTaskExecutor()->Set(task_executor);
 
   (*GetTaskExecutorMap())[extension_id - 1] = task_executor;
 }
@@ -50,16 +42,11 @@ void UnregisterTaskExecutorForTesting(uint8_t extension_id) {
   DCHECK_NE(extension_id, TaskTraitsExtensionStorage::kInvalidExtensionId);
   DCHECK_LE(extension_id, TaskTraitsExtensionStorage::kMaxExtensionId);
   DCHECK_NE((*GetTaskExecutorMap())[extension_id - 1], nullptr);
+
   (*GetTaskExecutorMap())[extension_id - 1] = nullptr;
 }
 
 TaskExecutor* GetRegisteredTaskExecutorForTraits(const TaskTraits& traits) {
-  if (traits.use_current_thread()) {
-    auto* current_task_executor = GetTLSForCurrentTaskExecutor()->Get();
-    DCHECK(current_task_executor);
-    return current_task_executor;
-  }
-
   uint8_t extension_id = traits.extension_id();
   if (extension_id != TaskTraitsExtensionStorage::kInvalidExtensionId) {
     TaskExecutor* executor = (*GetTaskExecutorMap())[extension_id - 1];