simplify dynamic_cast followed by static_cast

Change-Id: I965afcf05d1675094cfde53d3590a0fd00f26279
Reviewed-on: https://gerrit.libreoffice.org/44460Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>