[FIX] mrp: production chained to a done or cancelled move

Do not try to run `action_assign` on the next move if it is done or
cancelled. The issue is that `action_assign` will first unlink the
existing pack operations before creating new ones, and the system
forbids to unlink these ones.

To reproduce this issue:
1. Create a product, routes manufacturing and MTO
2. Create SO with that product
3. Go to DO and force assign then cancel the delivery
4. Go to Manufacturing order created and produce

opw 778897