[IMP] mrp, sale_mrp: planned date on MO

The purpose of this commit is to simplify the `mrp.production` model in
reducing the number of date field.

The following fields was kept:
    - `date_planned_start`: the planned date for the beginning of the MO
    - `date_planned_finished`: the planned date for the ending of the MO
    - `date_start_wo`: the preferred date to start a Work Order in case
      the MO will plan one.
    - `date_deadline`: is only indicative and is automatically set when
      the manufacturing order is needed for a sale order.
    - `date_start` and `date_finished`: no changes.

Task #1935229

Co-authored-by: Whenrow <whe@odoo.com>