Revert "Optimize Spree::InventoryUnit#quantity_no_less_than_shipped"

This reverts commit df0740c.

Conflicts:

	core/app/models/spree/line_item.rb

core/app/models/spree/line_item.rb

@@ -97,7 +97,7 @@ def stock_availability
       end
 
       def quantity_no_less_than_shipped
-        already_shipped = order.shipments.reduce(0) { |acc,s| acc + s.inventory_units.where(:variant_id => variant.id).count }
+        already_shipped = order.shipments.reduce(0) { |acc,s| acc + s.inventory_units.select { |i| i.variant == variant }.count }
         unless quantity >= already_shipped
           errors.add(:quantity, I18n.t('validation.cannot_be_less_than_shipped_units'))
         end