removed premature optimization for Product.is_group

This premature optimization causes most of the bottleneck in rendering of details page. It is not a good idea to load all variants.

Example product where you don't want to do that:
Levi's Men's 501 Jean
http://www.amazon.com/gp/product/B0018OR118

Colors: 37
Sizes: 87

Total Variants: 3,219