The posted script for YouTube is no longer functional. However, I found a new one that works perfectly fine. It deletes the videos one by one automatically.

setInterval(function() {
	document.querySelector('#contents button#button').click();
	var things = document.evaluate('//span[contains(text(),"Watch later")]',document,null,XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,null);
		for (var i = 0; i < things.snapshotLength; i++) {
		    things.snapshotItem(i).click();
		}
}, 1000);