Improve work order removal on sale quote

* Call detach_slave on the slave so it will be removed from the validation
  values. Without this, its last validation value would still be taken in
  consideration and, if it was invalid, would make it impossible for the user
  to go forward.

* If the model is new, we can remove the removed workorder from the database

Change-Id: I59947c19d90f92cc410b1f257e05dedddace0ff1