This reverts commit
436bbb0a48ff866925fe0a3987928ab554bc3c6d.
The many2many widget is usefull, as it allows to select an existing picking instead of creating a new one when clicking add an item from the list
<field name="user_id"/>
</group>
<separator string="Pickings"/>
- <field name="picking_ids" domain="[('state', 'not in', ('done', 'cancel'))]">
+ <field name="picking_ids" widget="many2many" domain="[('state', 'not in', ('done', 'cancel'))]">
<tree>
<field name="name"/>
<field name="backorder_id"/>