Extract private method

This was initially intended to cache the result of the `#map` and
`#uniq` calls but we're not confident enough and don't want to
scopecreep this. It's still worth to point out that this is what we
need, line items' `unique orders`. Hopefully, next time we find a way to
optimize it.

engines/order_management/app/services/order_management/reports/bulk_coop/bulk_coop_report.rb

@@ -159,15 +159,19 @@ module OrderManagement
         end
 
         def customer_payments_total_cost(line_items)
-          line_items.map(&:order).uniq.sum(&:total)
+          unique_orders(line_items).sum(&:total)
         end
 
         def customer_payments_amount_owed(line_items)
-          line_items.map(&:order).uniq.sum(&:outstanding_balance)
+          unique_orders(line_items).sum(&:outstanding_balance)
         end
 
         def customer_payments_amount_paid(line_items)
-          line_items.map(&:order).uniq.sum(&:payment_total)
+          unique_orders(line_items).sum(&:payment_total)
+        end
+
+        def unique_orders(line_items)
+          line_items.map(&:order).uniq
         end
 
         def empty_cell(_line_items)