[FIX] mrp: do not recompute duration expected

Since 2515482d, the duration expected of a workorder was always set
to the real duration at the validation of the workorder. This commit
ensure the duration expected is left unchanged even if the
date_planned_finished is updated.

closes odoo/odoo#108462

Opw: 3008089
X-original-commit: 61771743
Signed-off-by: William Henrotin (whe) <whe@odoo.com>