Remove string attribute from views

issue2350

view/employee_cost_price_form.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
-<form string="Cost Prices">
+<form>
     <label name="date"/>
     <field name="date"/>
     <label name="cost_price"/>

view/employee_cost_price_tree.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
-<tree string="Cost Prices">
+<tree>
     <field name="date"/>
     <field name="cost_price"/>
 </tree>