Each PieChart is an individual instance and won't interact with each other if not defined otherwise.
So in this case you need to create your own function that would iterate through all PieCharts and call selection([])
on them.
As we currently don't have an API method of getting all subcharts, then you need to use a hack for it.
It is not recommended as it is a subject to change in further versions, but in your case you can use it on your own risk to achieve your needed goal.
var chart = new GeoChart();
$("#btn").on("click", function(e) {
e.preventDefault();
var subs = chart._impl.events.subcharts;
for(var i=0; i < subs.length; i++) {
var s = subs[i]["api"];
console.log("s", s);
s.selection([]);
}
});
Working demo:
http://jsfiddle.net/ng3m9bwb/