[MBI] Revert: Make ThreadTaskRunnerHandle::Get() return currently active...

[MBI] Revert: Make ThreadTaskRunnerHandle::Get() return currently active AgentGroupSchedulerImpl's default task runer.

revert target:
https://chromium-review.googlesource.com/c/chromium/src/+/2413728

The reason of revert:
1. The previous CL probably relates to this bug report: https://bugs.chromium.org/p/chromium/issues/detail?id=1133071
2. We want to make sure of these two things.
  - Ensure that ThreadTaskRunnerHandle::Get return value is not stored in the place where it can be accessed by other AgentGroup (e.g. TLS, shared data structures, etc).
  - Make sure that ThreadTaskRunnerHandle::Get does not return per-thread task runner (as otherwise it can lead to propagation of incorrect AG attribution via chained TTRH::Get calls).

[related design doc]
https://docs.google.com/document/d/1y-vHkrD1z2RtyWYwT6rJkSLHClYNjDDpUbTtBU7l95A

Bug: 1133071, 1105403
Change-Id: Ifc47179c081bad95f1b8e96d7ef3581a7a64402a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437536Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tal Pressman <talp@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811605}

third_party/blink/renderer/platform/scheduler/main_thread/main_thread_scheduler_impl.cc

@@ -2363,14 +2363,6 @@ void MainThreadSchedulerImpl::SetCurrentAgentGroupScheduler(
     TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(
         TRACE_DISABLED_BY_DEFAULT("agent_scheduling_group"), "MTS_scope", this);
   }
-  if (agent_group_scheduler_impl) {
-    sequence_manager_->SetDefaultTaskRunner(
-        agent_group_scheduler_impl->DefaultTaskRunner());
-  } else {
-    // If there is no proper AgentGroupScheduler, it means that the
-    // current scheduler is MainThreadScheduler.
-    sequence_manager_->SetDefaultTaskRunner(helper_.DefaultTaskRunner());
-  }
 }
 
 std::unique_ptr<ThreadScheduler::RendererPauseHandle>