There is no property setting, but you can handle QueryCellStyleInfo and set the tip empty there for summary cells.
private void gridGroupingControl1_QueryCellStyleInfo(object sender, GridTableCellStyleInfoEventArgs e)
{
if (e.TableCellIdentity.TableCellType == GridTableCellType.GroupCaptionSummaryCell
|| e.TableCellIdentity.TableCellType == GridTableCellType.AnySummaryCell)
e.Style.CellTipText = "";
}