Remove unused headers in task_runner_util.h after fixing IWYU

Follow-up to https://chromium-review.googlesource.com/c/chromium/src/+/2076423
After https://chromium-review.googlesource.com/c/chromium/src/+/2087971
and https://chromium-review.googlesource.com/c/chromium/src/+/2087892

R=ajwong@chromium.org

Bug: 1026641
Change-Id: Ia2c180759677a51502c5c1c04508d165f7c35856
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087893
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747707}

base/task_runner_util.h

@@ -14,11 +14,6 @@
 #include "base/post_task_and_reply_with_result_internal.h"
 #include "base/task_runner.h"
 
-// TODO: Remove these unused includes after fixing IWYU usage in
-// affected files.
-#include "base/bind_helpers.h"
-#include "base/callback_helpers.h"
-
 namespace base {
 
 // When you have these methods