hidpi, startcenter: Don't set fixed font heigt in the startcenter.

Change-Id: I68efbd08d21b514ab1659cd6bddfc2c4788c4a74
Reviewed-on: https://gerrit.libreoffice.org/10682
Reviewed-by: Zolnai Tamás <tamas.zolnai@collabora.com>
Tested-by: Zolnai Tamás <tamas.zolnai@collabora.com>