Based on my test using ChartSeries_DataPoint.aspx sample, using OnAfterInitialize client side event handler, we could get the FilterExpression element based on the available filter info. However, the element will only be available after the pop up has been displayed, hence the setInterval, as shown in the snippet: 

function WebGrid1_OnAfterIntialize(id) 
{
    var grid = ISGetObject(id);

    setInterval(function () {
        try {
            var filterInfo = grid.CachePivotFilterInfoCollection[0];
            var filterExpElem = test.GetFilterExpressionElement();

            filterExpElem.parentNode.parentNode.style.display = "none";
        }
        catch (err) {}
                
    }, 150);
            
}