NetteLoader: removed dependency on Nette\Object, simplified loader.php

Another thing to consider is to remove `NetteLoader::getInstance()`. Does anybody need to use `NetteLoader` as a singleton? It was used to disable `NetteLoader` by `RobotLoader` but this is no longer required. However it is used now to fix compatibility with Nette 2.0 [in Kdyby](https://github.com/Kdyby/Autowired/blob/1723f8f51f667cee7fd3be6b5e537400b3599726/src/Kdyby/Autowired/DI/AutowiredExtension.php#L25-L28).