[16.0][FIX]sale_delivery_state: add state to silence warning

[PicchiSeba]

Both sale_delivery_state and sale_stock add a delivery_status field. The two modules add the same selection options except for one, missing in sale_delivery_state. This is the "started" option.

With both modules installed we are witnessing a lot of warnings like the following

sale.order.delivery_status: selection=[('pending', 'Not Delivered'), ('started', 'Started'), ('partial', 'Partially Delivered'), ('full', 'Fully Delivered')] overrides existing selection; use selection_add instead 

With this PR we add the same option as sale_stock but we ignore it during the computation.

[PicchiSeba]

question: given that both modules add the same field how do we ensure the execution of the right compute function when both of them are installed?