UPDATED 22.11.2022

It's been two years since the last update, so here's the updated working script as per the comments below.

Thanks to BryanHaley for this.

German version / Deutsche Version:

setInterval(function () {
    video = document.getElementsByTagName('ytd-playlist-video-renderer')[0];

    video.querySelector('#primary button[aria-label="Aktionsmenü"]').click();

    var things = document.evaluate(
        '//span[contains(text(),"Später ansehen")]',
        document,
        null,
        XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,
        null
    );

    for (var i = 0; i < things.snapshotLength; i++) 
    {
        things.snapshotItem(i).click();
    }
}, 500);

Non-english users will need to change "Action menu" and "Remove from" to what YouTube uses for their localization.