[FIX] hr_expense: Don't always recompute product cost

Fix a bug introduced by 67901a44

When an expense is submitted and for the steps after, there is no need
to recompute the product_cost as it may be confusing
or generate discrepancies with the account move by changing the totals.

task-3580004

closes odoo/odoo#141400

Signed-off-by: Olivier Colson (oco) <oco@odoo.com>