...so that it no longer has a m_aXMLAttributeNamespace member that the user-
provided copy ctor did not copy.  (Which was presumably by accident, but appears
to not have had bad consequences due to how XMLNamespaces::adNamespace is only
called before the copy ctor is called in SaxNamespaceFilter::startElement,
framework/source/fwe/xml/saxnamespacefilter.cxx).

Found by new -Wdeprecated-copy of GCC trunk towards GCC 9.

Change-Id: I0701ecdfbef9c078a09ed411f4d9ccd166271aae
Reviewed-on: https://gerrit.libreoffice.org/56469
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>