Remove useless BootManager::Stop

It's not used anywhere other than in DolphinQt2, where the usage is
incorrect and stupid since we shouldn't be trying to stop the core
and 'restore config' that was changed by the core at app exit time,
but immediately when the core is being shut down.

Source/Core/DolphinQt2/Main.cpp

@@ -220,7 +220,6 @@ int main(int argc, char* argv[])
     retval = app.exec();
   }
 
-  BootManager::Stop();
   Core::Shutdown();
   UICommon::Shutdown();
   Host::GetInstance()->deleteLater();