[FIX] stock_account: value of return

When returning a delivery where the value was taken from different
receipt, the last receipt's unit cost was wrongly used when returning
the delivery.

Now we use the `unit_cost` of the delivery when returning it.

task-2150889

closes odoo/odoo#41422

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>