Solve the environment conflict with Cocos.

@ricardoquesada, @minggo 
Please review this PR. I removed the logic of write environment `COCOS_X_ROOT` in `setup.py`.

Because `Cocos` depends on the environment to finding the prebuilt libs. Developers will encountered problems when they following these steps:
1. Download & Install Cocos.
2. Create a project from Cocos.
3. Download & Install cocos2d-x.
4. Invoke `setup.py` in cocos2d-x.
5. The project created from Cocos CAN NOT build on platforms: Android & Win32.

So I think the `setup.py` of cocos2d-x should not write the environment `COCOS_X_ROOT`.