Improve available budget calculation

Don't store individual discounts, and instead sum them up directly.

lib/planet_express/discounts/discount_budget.rb

@@ -2,16 +2,16 @@
 
 module PlanetExpress
   class DiscountBudget
-    attr_reader :budget
+    attr_reader :budget, :available_budget
 
     def initialize(budget:)
       @budget = budget
-      @applied_discounts = []
+      @available_budget = budget
     end
 
     def register_applied_discount(discount_amount)
       possible_discount = maximum_possible_discount(discount_amount)
-      applied_discounts.push(possible_discount)
+      @available_budget = available_budget - possible_discount
       possible_discount
     end
 
@@ -19,20 +19,8 @@ module PlanetExpress
       [available_budget, discount_amount].min
     end
 
-    def available_budget
-      budget - spent_budget
-    end
-
     def remaining_budget?
       !available_budget.zero?
     end
-
-    private
-
-    attr_reader :applied_discounts
-
-    def spent_budget
-      applied_discounts.sum
-    end
   end
 end