Refactor definition of environment.promotions preference
Refactor definition of environment.promotions preference

There's no need to redefine it as Spree::Core::Environment::Promotions
since it's already that thing, see it's initializer in
core/lib/spree/core/environment.rb and that's exactly what we
are already doing with calculators.

pervino/solidus,pervino/solidus,pervino/solidus,pervino/solidus