fdo#69613 DOCX export: Fix for '\o' flag field should export once.

(This is partial fix for this bug)
Description:
TOC flag field '\o' exported twice after roundtrip because the export logic for '\o' is repeated in ww8atr.cxx.

XML file difference
In document.xml
Before :
    <w:instrText>
     TOC \o "1-3" \o "1-3" \h
    </w:instrText>

After :
    <w:instrText>
     TOC \o "1-3" \h
    </w:instrText>

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/7206

Change-Id: I136744927b8d5a48869a2bfc87278bede697c655