Remove non need belongs_to associations from Adjustments

It turns out the "tax_rate" association isn't used and wasn't working.
Same for the "voucher" one, which I added to be consistent with existing
code.
Both of these weren't caught by the specs because you can't test associations
with a custome relation with 'shouda-matchers' see: thoughtbot/shoulda-matchers#981

app/models/spree/adjustment.rb

@@ -42,11 +42,6 @@ class Adjustment < ApplicationRecord
     belongs_to :order, class_name: "Spree::Order"
     belongs_to :tax_category, class_name: 'Spree::TaxCategory'
 
-    belongs_to :tax_rate, -> { where spree_adjustments: { originator_type: 'Spree::TaxRate' } },
-               foreign_key: 'originator_id'
-    belongs_to :voucher, -> { where spree_adjustments: { originator_type: 'Voucher' } },
-               foreign_key: 'originator_id', inverse_of: :adjustments
-
     validates :label, presence: true
     validates :amount, numericality: true
 

spec/models/spree/adjustment_spec.rb

@@ -11,14 +11,10 @@ module Spree
       it { is_expected.to have_one(:metadata) }
       it { is_expected.to have_many(:adjustments) }
 
-      it { is_expected.to belong_to(:adjustable) }
-
       it { is_expected.to belong_to(:adjustable) }
       it { is_expected.to belong_to(:originator) }
       it { is_expected.to belong_to(:order) }
       it { is_expected.to belong_to(:tax_category) }
-      it { is_expected.to belong_to(:tax_rate) }
-      it { is_expected.to belong_to(:voucher) }
     end
 
     describe "scopes" do