the subclass field is just a copy of the parent field, so drop the
subclass, and consequently simply the CreateCanvases method, we don't
need to pass the parameter down, can just access the parent field.

Change-Id: I3be448a06b32d01cdda4fc36b78cafb4488e2913
Reviewed-on: https://gerrit.libreoffice.org/48553Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>