There's no method on the Sketch widget to delete a particular or selected graphic.
One posible option is to do it manually using the updateGraphics property of the widget which is a Collection of the graphics that are being edited/selected in that moment.
With that information we can use the method removeMany() from the "graphics" property(Collection) of your GraphicsLayer to remove them.
...
function deleteGraphics() {
const selectedGraphics = sketchWidget.updateGraphics; //Get selected graphics from the widget
if (selectedGraphics) {
graphicsLayer.graphics.removeMany(selectedGraphics); //We remove them from our layer
}
sketchWidget.complete(); //We "complete" the "update" event, setting updateGraphics to empty
}
...
If someone else has a better way to do it I will be happy if its posted, since I'm new to the ArcGIS JS API.