[python] don't include site-packages in python path

This is very error prone as add-on may 'silently work' if required dependence are installed system-wide. It's basically impossible to test if your add-on really works without a clean python install.

I have no idea why site-packages was [added](https://github.com/xbmc/xbmc/commit/c4fd22dd326af8a82c50d6f7d0df85a6518aa14b) in the first place. May completely break something.


sadly it won't work on debian based distros either, as those use /usr/lib/python2.7/dist-packages for system installed modules. Only manually compiled ones go to site-packages.
see https://wiki.debian.org/Python
'''dist-packages instead of site-packages. Third party Python software installed from Debian packages goes into dist-packages, not site-packages. This is to reduce conflict between the system Python, and any from-source Python build you might install manually. 
```