Clear invalid payments

When going back and changing the payment option, the previous payment gets invalidated.

app/controllers/split_checkout_controller.rb

@@ -51,6 +51,7 @@ class SplitCheckoutController < ::BaseController
 
   def update
     if confirm_order || update_order
+      clear_invalid_payments
       advance_order_state
       redirect_to_step
     else
@@ -69,6 +70,10 @@ class SplitCheckoutController < ::BaseController
 
   private
 
+  def clear_invalid_payments
+    @order.payments.with_state(:invalid).delete_all
+  end
+
   def confirm_order
     return unless @order.confirmation? && params[:confirm_order]