You can simply use showInLegend set to false: https://api.highcharts.com/highcharts/series.pie.showInLegend

You can modify your config like so:

{
    name: 'acknowledged',
    y: acknowledgedWarnings,
    color: 'grey',
    showInLegend: false // Hide this from the legend
}, ...

You can still use itemClick for interaction if needed.