[MBI] Remove GetCurrent() and SetCurrent() static functions from AgentGroupSchedulerImpl

GetCurrent() and SetCurrent() static functions are not used
anymore because these functions are moved to MainThreadSchedulerImpl. So this CL removes these static functions.

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

Bug: 1105403
Change-Id: I3def70869cd97cf6b2f06642630c1c707d28ae3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437650Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tal Pressman <talp@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811580}

third_party/blink/renderer/platform/scheduler/main_thread/agent_group_scheduler_impl.h

@@ -23,8 +23,6 @@ class MainThreadTaskQueue;
 class PLATFORM_EXPORT AgentGroupSchedulerImpl
     : public blink::AgentGroupScheduler {
  public:
-  static AgentGroupSchedulerImpl* GetCurrent();
-  static void SetCurrent(AgentGroupSchedulerImpl*);
   explicit AgentGroupSchedulerImpl(
       MainThreadSchedulerImpl& main_thread_scheduler);
   AgentGroupSchedulerImpl(const AgentGroupSchedulerImpl&) = delete;