DOCX export: avoid duplicated w:sz child elements

sw/qa/extras/ooxmlexport/data/zoom.docx is a reproducer for this,
DocxAttributeOutput::CharFontSizeCJK() is just a wrapper around
CharFontSize(), and both produce a <w:sz> element, but to produce valid
OOXML, only one child element should be written, so ignore the second
one.

Change-Id: I7474fccadccf573f706c9b148b7e450c39ad5efb