[Bf-cycles] Remove old Sky Model

[Thomas Dinges]

Hi all,
As part of my GSoC 2013 I implemented the Hosek / Wilkie sky model, 
which is more realistic than the older Preetham model.

Now, more than 2 years later, I wonder if people still use the old 
model, as I'd like to remove the old one.

Any strong reasons to keep the old one?

Best regards,
Thomas