combo-row: Don't change row sensitivity on model change
AdwComboRow becomes sensitive or insensitive automatically when the model changes. In particular, it is forced sensitive when the model contains more than 0 items.
This can mess with the user-set sensitivity of the row if the model changes. See gnome-control-center#3122 (closed).
So let's remove that sensitivity management althogether. The row is anyways not activatable once it's empty and so can't be interacted with furter.