There is 4 steps to undo this :

1) Make the “state” field appears in the picking form view.

Go to Warehouse → Incoming Shipments and then activate the developer mode.

Click on the shipment you wanna delete, then click on “Debug View#xxx” and choose “Edit FormView”.You have to add this line of code in the form view :

<field name="state" readonly="0" invisible="0"/>

Admin rights are required to do this.

2) Edit and change the state to “draft”

3) Delete the shipment

4) Delete the line of code that you previously add in the form view.