Here's that code written by someone who has used Javascript in the last 10 years:

    (function () {
        for (const node of document.querySelectorAll('body *')) {
            if (['fixed', 'sticky'].includes(getComputedStyle(node).position)) {
                node.style.display = 'none'
                // node.remove()
            }
        }
    })()

I found tree mutation too aggressive in the first site I tested it on (the site's JS code expected the navbar to exist, so it just blew up with runtime errors). Simply changing the display property is sufficient.