Remove icon attribute on button in move tree view

The changeset 38cc2d49d77f added it but such attribute is not allowed
in tree view.

view/move_tree.xml

@@ -11,7 +11,7 @@ this repository contains the full copyright notices and license terms. -->
     <field name="planned_date"/>
     <field name="effective_date"/>
     <field name="state"/>
-    <button name="cancel" string="Cancel" icon="tryton-cancel"/>
-    <button name="draft" string="Reset to Draft" icon="tryton-clear"/>
+    <button name="cancel" string="Cancel"/>
+    <button name="draft" string="Reset to Draft"/>
     <field name="unit_digits" tree_invisible="1"/>
 </tree>