[18.0] sale_discount_display_amount: Wrong discount_subtotal when resetting discount to 0

[iggups]

Module

sale_discount_display_amount

Describe the bug

After setting a percentage discount via the 'sale.order.discount' wizard and resetting it to 0, the value of 'discount_subtotal' is not updated correctly

To Reproduce

Affected versions: 18.0

Steps to reproduce the behavior:

1. Click on the "Discount" button in the sales order
2. Apply a discount "On all Order Lines" with 1%
3. discount_subtotal is updated
4. Click on the "Discount" button in the sales order
5. Apply a discount "On all Order Lines" with 0%
6. discount_subtotal stays on the value from step 2

Expected behavior
The field resets to 0, because no discount is present

Additional context