No need to reflect ongoing compatible changes in type_reference

Had been added as part of 7a8ed362 "[API CHANGE]
revert and deprecate *BackGraphicURL add *BackGraphic", for no apparent reason.
(Will eventually be added with a "Update the reference rdbs to
libreoffice-6-1-0" commit.)

Change-Id: If6c0405df91accc52a67ad62d7ee7dfb49a4c569
Reviewed-on: https://gerrit.libreoffice.org/53981Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

offapi/type_reference/offapi.idl

@@ -3643,7 +3643,6 @@ module com {
      [property, maybevoid, optional] boolean ParaIsConnectBorder;
      [property, optional] string ListId;
      [property, optional] short OutlineLevel;
-     [property, optional] ::com::sun::star::graphic::XGraphic ParaBackGraphic;
     };
    };
    module util {