it's not adding anything useful, just hold the underlying SalBitmap
instead

Change-Id: I54852707b2f8af99283b9c882a428a8a7a11c4cf
Reviewed-on: https://gerrit.libreoffice.org/52955Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>