fix(GridFieldAddNewMultiClass) removed obsolete config param
fix(GridFieldAddNewMultiClass) removed obsolete config param

FIRST_SET has been removed from SS4
silverstripe-australia/silverstripe-gridfieldextensions,symbiote/silverstripe-gridfieldextensions,dhensby/silverstripe-gridfieldextensions,kinglozzer/silverstripe-gridfieldextensions,silverstripe-australia/silverstripe-gridfieldextensions,symbiote/silverstripe-gridfieldextensions,kinglozzer/silverstripe-gridfieldextensions,dhensby/silverstripe-gridfieldextensions