Improve OrdersHelper spec and delete dead code

The removed test here was checking for adjustments that have an amount of zero and are eligible. If the amount is zero, it will already be marked as ineligible.

spec/helpers/admin/orders_helper_spec.rb

@@ -7,19 +7,13 @@
     let(:order) { create(:order) }
 
     it "selects eligible adjustments" do
-      adjustment = create(:adjustment, adjustable: order, amount: 1)
+      adjustment = create(:adjustment, order: order, adjustable: order, amount: 1)
 
       expect(helper.order_adjustments_for_display(order)).to eq [adjustment]
     end
 
     it "filters shipping method adjustments" do
-      create(:adjustment, adjustable: order, amount: 1, originator_type: "Spree::ShippingMethod")
-
-      expect(helper.order_adjustments_for_display(order)).to eq []
-    end
-
-    it "filters zero tax rate adjustments" do
-      create(:adjustment, adjustable: order, amount: 0, originator_type: "Spree::TaxRate")
+      create(:adjustment, order: order, adjustable: order, amount: 1, originator_type: "Spree::ShippingMethod")
 
       expect(helper.order_adjustments_for_display(order)).to eq []
     end