1. Open your watch later playlist on youtube.
2. Open the development console for your browser ( Ctrl+Shift+J for chrome )
3. paste this script into the console

var lastNumVideos

function removeOne() {
	var numVideos = document.querySelectorAll('.pl-video-edit-remove').length
	if (numVideos === lastNumVideos) {
		return
	}
	if (numVideos < 10) {
		document.querySelector('.browse-items-load-more-button').click()
   }
	document.querySelector('.pl-video-edit-remove').click()
	lastNum = numVideos
}
setInterval(removeOne, 30)

4. Press the enter key
5. Watch your watch later playlist empty in realtime :D

If you need to stop the script simply close or refresh the playlists's tab in your browser.

Enjoy! :D

The script can be left open forever without flooding youtube with requests, it will just make lots of unnecessary DOM queries, which shouldn't be a real problem.