remove using namespace ::chart::ContainerHelper;
to match other uses

Change-Id: I343086a6b7d70c84499b209680973431c7317219
Reviewed-on: https://gerrit.libreoffice.org/27184Reviewed-by: Jochen Nitschke <j.nitschke+logerrit@ok.de>
Tested-by: Jenkins <ci@libreoffice.org>