[FIX] stock: remove duplicate field

On the stock picking form, the 'Effective date' and the 'Completion date'
are technically the same field. This commit remove the second one.

Related to Task: 1886591

addons/stock/views/stock_picking_views.xml

@@ -326,7 +326,6 @@
                                 <group>
                                     <field name="picking_type_code" invisible="1"/>
                                     <field name="move_type" attrs="{'invisible': [('picking_type_code', '=', 'incoming')]}"/>
-                                    <field name="date_done" string="Completion Date" readonly="1" attrs="{'invisible': [('state', '!=', 'done')]}"/>
                                 </group>
                                 <group>
                                     <field name="company_id" groups="base.group_multi_company" options="{'no_create': True}"/>