[17.0][FIX] purchase_manual_delivery: always call the full confirmation stack

[len-foss]

Suppose you have module purchase_x that depends on purchase, and that it does an override of button_confirm to call fx.
Because purchase_x and purchase_manual_delivery have no relation to each other, we could either have purchase_manual_delivery => purchase_x => purchase, or purchase_x => purchase_manual_delivery => purchase.

If you call button confirm, in all cases you will call fx once. If you call button_confirm_manual, in one case you will call fx but not the other, depending on how the ORM loaded modules. If you decide that you want an easy solution and add an override of button_confirm_manual to also call fx, then you have the possibility to call fx twice, so you'd better be sure to make it idempotent.