Tag filter of an AttributeExternalKey may be redefined

[xtophe38]

Nothing forbids the filter of an AttributeExternalKey to be redefined by an extension. But this is denied by iTop-schema at the moment and an error is displayed.

For instance:
<field id="functionalci_id" xsi:type="AttributeExternalKey" _delta="must_exist">
<filter _delta="redefine"><![CDATA[SELECT FunctionalCI WHERE finalclass IN ('NetworkDevice', 'Server', 'VirtualMachine')]]></filter>
</field>

=> Attribute _delta is not allowed here.