[FIX] sale: small cleanup of line computations

Fine-tuning of 3e61dab2.
While it worked, it was not fixing the right issue.
However the test does make sure that the root cause has been fixed,
but it missed the mark on not knowing what it was testing, and subsequently
having useless stuff and comments that were downright misleading.

closes odoo/odoo#38258

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>