Deleting all comments at once on a PDF (DC)?

Open the Comments panel, click into the comments list, press Ctrl+A, press Delete.

Alternatively, you can run this code from the JS Console or even as an Action or Custom Command:

this.syncAnnotScan();
var annots = this.getAnnots();
if (annots!=null) {
	for (var i=annots.length-1; i>=0; i--) {
		annots[i].destroy();
	}
}