Omit XML namespace declaration for <iq/> stanzas

Only the child elements of `<iq/>` stanzas are qualified by non-default namespaces, not the `<iq/>` stanzas themselves.

This change just clarifies the code.  It doesn't alter the behaviour, as the `<iq/>` stanzas are handed over to `jlib:iq_to_xml/1`, and that function [ignores][1] the `xmlns` attribute anyway.

[1]: https://github.com/processone/ejabberd/blob/6baf3a24de429ed6cf27b267d5b91ef3d64114bf/src/jlib.erl#L453