MAGETWO-91511: Top destinations cannot be removed after a selection was previously saved

- Adding can_be_empty option to destinations field

Author: vprohorov

app/code/Magento/Backend/etc/adminhtml/system.xml

@@ -234,6 +234,7 @@
                 <field id="destinations" translate="label" type="multiselect" sortOrder="40" showInDefault="1" showInWebsite="1" showInStore="1">
                     <label>Top destinations</label>
                     <source_model>Magento\Directory\Model\Config\Source\Country</source_model>
+                    <can_be_empty>1</can_be_empty>
                 </field>
             </group>
             <group id="locale" translate="label" type="text" sortOrder="8" showInDefault="1" showInWebsite="1" showInStore="1">