Export LineStyle_NONE instead of default linestyle of PlotArea border,
because LibreOffice make invisible the Wall shape properties in case of
PIE/NET/FILLED_NET charts. Or other solution in the future is set the
default LineStyle of these charts to LineStyle_NONE.

Change-Id: I59e6f18058713f4a0d4c77da8760a319f9c25152
Reviewed-on: https://gerrit.libreoffice.org/61547
Tested-by: Jenkins
Reviewed-by: László Németh <nemeth@numbertext.org>