UICommon: Remove global system accessor in TriggerSTMPowerEvent()

This can be passed through via the IOS instance.

Source/Core/UICommon/UICommon.cpp

@@ -492,8 +492,7 @@ bool TriggerSTMPowerEvent()
     return false;
 
   Core::DisplayMessage("Shutting down", 30000);
-  auto& system = Core::System::GetInstance();
+  ios->GetSystem().GetProcessorInterface().PowerButton_Tap();
-  system.GetProcessorInterface().PowerButton_Tap();
 
   return true;
 }