Update create_adjustment to create with an amount of 0

Amount calculation is handled by VoucherAdjustmentService.calculate

app/models/voucher.rb

@@ -21,12 +21,11 @@ def display_value
   # but vouchers have complicated calculation so we can't easily use Spree::Calculator. We keep
   # the same method to stay as consistent as possible.
   #
-  # Creates a new voucher adjustment for the given order
+  # Creates a new voucher adjustment for the given order with an amount of 0
+  # The amount will be calculated via VoucherAdjustmentsService.calculate
   def create_adjustment(label, order)
-    amount = compute_amount(order)
-
     adjustment_attributes = {
-      amount: amount,
+      amount: 0,
       originator: self,
       order: order,
       label: label,

spec/models/voucher_spec.rb

@@ -45,8 +45,8 @@
     let(:voucher) { create(:voucher, code: 'new_code', enterprise: enterprise, amount: 25) }
     let(:order) { create(:order_with_line_items, line_items_count: 3, distributor: enterprise) }
 
-    it 'includes the full voucher amount' do
-      expect(adjustment.amount.to_f).to eq(-25.0)
+    it 'includes an amount of 0' do
+      expect(adjustment.amount.to_f).to eq(0.0)
     end
 
     it 'has no included_tax' do