legendoption has no property pieSliceText. pieSliceText- selection of the entire pie chart. It seems that there is no way to disable% in the legend, but you can completely disable the legend using:
legend: {
position: 'none'
}
and create your own. How this is done is described in the answer How to add Percentage and Total according to the legend of Google graphic diagrams . You can display your own text with additional information and style.