[FIX] mrp: issue with products with variants

Step to follow

- Create a product with 2 variants and set route to Manufacture
- Create a BOM for the product
- Create an MO for one of the variants
- Save (in Draft State)
- Edit
- Change to the other variant
- Confirm
- Finish the MO Order
- Look at product moves on MO

Cause of the issue

In MrpProduction._onchange_move_finished method, self.move_finished_ids
was not empty before being assign.

Solution

Removed the records from self.move_finished_ids before
 assign another one to it.

opw-2572644

closes odoo/odoo#72909

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>