Omit XML namespace declaration for <iq/> stanzas

Only the child elements of <iq/> stanzas are qualified by the namespaces
in question, not the <iq/> stanzas themselves.

This change just clarifies the code.  It doesn't alter the behaviour, as
those <iq/> stanzas are handed over to jlib:iq_to_xml/1, and that
function ignores the 'xmlns' attribute anyway.
Merge pull request #218 from weiss/omit-iq-xmlns

Omit XML namespace declaration for <iq/> stanzas