Fix payment_total calculation

For payment that complete during the checkout (Paypal, Stripe) the
amount was recorded twice against `order.payment_total`. This is because
the `payment_total` gets updated in an afer_save Payment callback when a
payment is completed, and then once more when we process payment in
`Spree::Order#process_each_payment`.
This is an existing  issue on master, but it was hidden by the
`update_shipping_fees!` callback, it trigerred an update of the order's
total, which then updated `order.payment_total` with the correct value.
Now that we removed the callback, the bug showed up.

Note, I updated the stripe specs for consistency even though they are
currently disabled.

app/models/spree/order.rb

@@ -673,10 +673,6 @@ module Spree
         break if payment_total >= total
 
         yield payment
-
-        if payment.completed?
-          self.payment_total += payment.amount
-        end
       end
     end
 

app/views/spree/orders/_totals_footer.html.haml

@@ -29,7 +29,7 @@
       %td.text-right{colspan: "3"}
         %strong
           = t :order_amount_paid
-      %td.text-right.total
+      %td.text-right.total{id: "amount-paid"}
         %strong
           = order.display_payment_total.to_html
   - if order.outstanding_balance.positive?

spec/models/spree/order_spec.rb

@@ -294,15 +294,6 @@ RSpec.describe Spree::Order do
         expect(payment).to receive(:process!)
         expect(order.process_payments!).to be_truthy
       end
-
-      it "stores the payment total on the order" do
-        allow(payment).to receive(:process!)
-        allow(payment).to receive(:completed?).and_return(true)
-
-        order.process_payments!
-
-        expect(order.payment_total).to eq(payment.amount)
-      end
     end
 
     context "when a payment raises a GatewayError" do

spec/system/consumer/shopping/checkout_paypal_spec.rb

@@ -69,6 +69,7 @@ RSpec.describe "Check out with Paypal" do
 
         click_on "Complete order"
         expect(page).to have_content "Your order has been processed successfully"
+        expect(page.find("#amount-paid").text).to have_content "$19.99"
 
         expect(order.reload.state).to eq "complete"
         expect(order.payments.count).to eq 1

spec/system/consumer/shopping/checkout_stripe_spec.rb

@@ -71,6 +71,8 @@ RSpec.describe "Check out with Stripe" do
           checkout_with_stripe
 
           expect(page).to have_content "Confirmed"
+          expect(page.find("#amount-paid").text).to have_content "$19.99"
+
           expect(order.reload.completed?).to eq true
           expect(order.payments.first.state).to eq "completed"
         end