[ESP32] Do not delete default event loop

The PlatformManagerImpl::_InitChipStack() does not create default event
loop, so the PlatformManagerImpl::_Shutdown() shall not delete it.

src/platform/ESP32/PlatformManagerImpl.cpp

@@ -107,7 +107,7 @@ void PlatformManagerImpl::_Shutdown()
 
     Internal::GenericPlatformManagerImpl_FreeRTOS<PlatformManagerImpl>::_Shutdown();
 
-    esp_event_loop_delete_default();
+    esp_event_handler_unregister(IP_EVENT, ESP_EVENT_ANY_ID, PlatformManagerImpl::HandleESPSystemEvent);
 }
 
 void PlatformManagerImpl::HandleESPSystemEvent(void * arg, esp_event_base_t eventBase, int32_t eventId, void * eventData)