PowerPointImport_getSupportedServiceNames() was copy&pasted from
oox::core::FilterBase::getSupportedServiceNames(), so it can go away entirely.

Change-Id: Ia6352cea1c2d65ba94cae66b500de951d82f455a
Reviewed-on: https://gerrit.libreoffice.org/60124Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins