Removed experimental feature check.
Single slide exporting works as for multi slides exporting.

SVGExportTextDecorations unit test - fixed wrong XPath
The new way single slide are exported caused the xpath to the slide
group to be wrong,

Change-Id: I4db4dc77de4f0d1208418455e1fca3b5732c8477
Reviewed-on: https://gerrit.libreoffice.org/20459Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Andras Timar <andras.timar@collabora.com>