[FIX] stock: dont reassign package when removed in 3-steps delivery

In a 3-steps delivery with backorders, when removing the package in the 2nd step, it should be absent from the last step (OUT picking).
However, assigning a package on the backorder of the 1st picking and creating a new backorder reassigns the package to the OUT picking.
Settings: 3-steps delivery, packaging
Steps:
- create a SO of product A for 10 units
- on PICK_1 picking: put 2 units in Done and put in a package, validate, create a backorder
- on PACK_1 picking: remove the destination package for the 2 units, validate, create a backorder
- on OUT picking: the stock.move.line does not have a package
- on PICK_2 picking: put 2 units in Done and put in a package, validate, create a backorder
- on PACK_1 picking: the package was reassigned to the destination package
- on OUT picking: the stock.move.line has the original package that was removed before

This fix ensures that a package can't be assigned on 'done' stock.move.line

closes odoo/odoo#123575

X-original-commit: 863dc77f3034328ee0e85848e7e673b2de0211c9
Signed-off-by: Diez Stéphane (snd) <snd@odoo.com>