Kernel: Remove a few global instances from the kernel.

src/core/hle/kernel/thread.cpp

@@ -355,7 +355,7 @@ void Thread::SetActivity(ThreadActivity value) {
         // Set status if not waiting
         if (status == ThreadStatus::Ready || status == ThreadStatus::Running) {
             SetStatus(ThreadStatus::Paused);
-            Core::System::GetInstance().PrepareReschedule(processor_id);
+            kernel.PrepareReschedule(processor_id);
         }
     } else if (status == ThreadStatus::Paused) {
         // Ready to reschedule

src/core/hle/kernel/wait_object.cpp

@@ -95,7 +95,7 @@ void WaitObject::WakeupWaitingThread(std::shared_ptr<Thread> thread) {
     }
     if (resume) {
         thread->ResumeFromWait();
-        Core::System::GetInstance().PrepareReschedule(thread->GetProcessorID());
+        kernel.PrepareReschedule(thread->GetProcessorID());
     }
 }