[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.