Indeed it is lacking a feature to remove all bookmarks. What I did is open the dev console and ran this script a couple of times:

// select all duplicateCards
let duplicateCards = document.querySelectorAll('.duplicate');
// click the checkboxes leaving one out, 500 at a time to avoid timing out (I had a lot of duplicates)
for (let i=0; i<500; i++) {
let boxes = duplicateCards[i].querySelectorAll('.custom-checkbox input');
for (let j=1; j boxes[j].click();
}
}
// manually click the `remove all selected` button
// repeat :-p