[REM] product: Remove product_manager field

The product_manager field is existing since v6.0 and never displayed in any views. It is used in sale_timesheet module. A task created from a procurement_order (if it is a service and we track the service) is assigned to the product manager. It doesn't make a lot of sense as the value should be set directly in the database. We will remove the field and assign the current user to the task instead, which is the common behavior in Odoo.