[FIX] stock: avoid recomputing qty_to_order when not in scheduler

Configure a reorder rule and a restock route for a product A
Go to Inventory / Operations / Replenishment
Remove all filters, find product A and under column 'To Order' input a
quantity, then hit the button 'Order Once'

The user quantity will be overwritten by the '_compute_qty_to_order'.
This is a fine tuning of f5054446

opw-2375317

closes odoo/odoo#61952

X-original-commit: 0d4ab53b
Signed-off-by: agr-odoo <agr-odoo@users.noreply.github.com>
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>