I know how to disable it, but sometimes I see my own created tooltip, but also another one. I think this is a automatically created alt tag, see the image below. Because this alt-tag already exist you see double information. So I could delete my tooltips, because Mendix already creates an alt-tag. And in 2.4 I don't see alt-tags.
To disable tooltip you have to click on each column with the property setting "show tooltip" --> true, and set it to "false". Then you have to select the datagrid and set the property "tooltip form" to 'None'. From my point of view, the tooltip configuration hasn't been changed compare to version 2.4.