[REV] pos_loyalty: revert gift card specifid point computation

The correct behavior for the gift cards should be that the value of the gift card is equal to the value of the order line.
For example :
- 50$ giftcard -> Apply discount 50% on the order line -> 25$ gift card
If we want to apply a discount on a gift card we need to use the global discount

The commit we are reverting was forcing the value of the gift card to be the base value of the order line.

This reverts commit 83ceeecde76a859749d0659d6c387d95155c8425.

OPW-3264945

closes odoo/odoo#120579

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>