Samples shipping category create created duplicate

Spree::ShippingCategory.create!(:name => "Default") is run in the [core
migration](https://github.com/spree/spree/blob/master/core/db/migrate/20
130830001033_add_shipping_category_to_shipping_methods_and_products.rb)
therefore it is not needed in the spree_sample:load rake task otherwise
it causes a duplicate.