<?php

$string = 'Texto de teste <i class="fab fa-accusoft">xxx</i> Teste Lorem Ipsim <i class="fab fa-accusoft">xxx</i>';
$doc = new DOMDocument();
$doc->loadHTML($string);

$selector = new DOMXPath($doc);
foreach($selector->query('//i[contains(attribute::class, "fa-")]') as $e){
    $e->parentNode->removeChild($e);
}

$string = $doc->saveHTML();

echo $string;