Since there is also no other information, like a label. Also
removes one unneeded level of layout indirection.

I'm not sure about the root placeholder child added by glade, but
other dialogs have it too, so keep it.

Change-Id: I235cc1ab5ea9ef37079353ee150eb000cb665dd1
Reviewed-on: https://gerrit.libreoffice.org/73075
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>