XData: fix adhoc command FORM_TYPE duplication

[stokito]

When sending adhoc command that have a form the FORM_TYPE field is also duplicated on submitting the form. For example for "User add"

<iq to="montague.net" type="set" id="42">
<command xmlns="http://jabber.org/protocol/commands" sessionid="42" node="http://jabber.org/protocol/admin#add-user">
<x xmlns="jabber:x:data" type="submit">
<field var="FORM_TYPE" type="hidden">
<value>http://jabber.org/protocol/admin</value>
</field>
<field var="FORM_TYPE" type="hidden">
<value>http://jabber.org/protocol/admin</value>
</field>
<field var="accountjid" type="jid-single">
<value>romeo@montague.net</value>
</field>
<field var="password" type="text-private">
<value>1234</value>
</field>
<field var="password-verify" type="text-private">
<value>1234</value>
</field>
</x>
</command>
</iq>

As a solution we can omit the FORM_TYPE when setting fields for a form.
This fix this and potentially other places that we didn't found.