IMPORTANT: This will delete ALL your videos on that playlist, so you can start fresh. Before moving on make sure that is exactly what you want.
Since that operation will take 1 second for each item you want to delete, I recommend detaching a tab from you current browser session before moving on.
Activate the javascript console with and type the following javascript snippet:
# YouTube's english version:
setInterval(function() {
document.querySelector('#contents button[aria-label="Action menu"]').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);
# versão em português brasil do youtube
setInterval(function() {
document.querySelector('#contents button[aria-label="Menu de ações"]').click();
var things = document.evaluate('//span[contains(text(),"Assistir mais tarde")]',document,null,XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,null);
for (var i = 0; i < things.snapshotLength; i++) {
things.snapshotItem(i).click();
}
}, 1000);