[FIX] stock: do not set user_id on prepare_invoice_group

The user_id is already set by the prepare_invoice method, which is called before the prepare_invoice_group (the user_id is already set, thus)
Besides, _prepare_invoice is overriden in sale_stock, to set the picking sale order salesman as user_id, and, without this correct, grouping invoicse by partner re-set the user_id to uid, which is wrong.