Deprecate Order#fully_discounted?
Deprecate Order#fully_discounted?

This method is unused anywhere else in Solidus itself, and should be
implemented at the store level. Additionally, the current implementation
is not correct.

Related discussion: #481

jordan-brough/solidus,forkata/solidus,jordan-brough/solidus,pervino/solidus,richardnuno/solidus,richardnuno/solidus,richardnuno/solidus,scottcrawford03/solidus,Arpsara/solidus,scottcrawford03/solidus,forkata/solidus,forkata/solidus,Arpsara/solidus,Arpsara/solidus,richardnuno/solidus,scottcrawford03/solidus,forkata/solidus,pervino/solidus,jordan-brough/solidus,scottcrawford03/solidus,pervino/solidus,pervino/solidus,Arpsara/solidus,jordan-brough/solidus