[FIX] purchase: width in tree view

In enterprise/web/static/src/xml/base.xml, the javascript template
ListView uses on each column (field) column.width() where width is
a javascript function. Then argument width cannot be use in tree view.

opw:667465

addons/purchase/purchase_view.xml

@@ -414,7 +414,7 @@
                     <field name="product_qty"/>
                     <field name="product_uom" groups="product.group_uom"/>
                     <field name="price_subtotal" widget="monetary"/>
-                    <field name="date_planned"  widget="date" width="135"/>
+                    <field name="date_planned"  widget="date"/>
                 </tree>
             </field>
         </record>