Clean Selected Option

$(".tendina-filtro option:selected").each(function() {
     $(this).removeAttr("selected");
});
$(".tendina-filtro option:first").each(function(){
    $(this).attr("selected","selected");
});

<select class="tendina-filtro" id="rentCustomer" name="rentNameCust" required="">
    <option data-identity="1">Mario </option>
    <option data-identity="2">Massimo </option>
    <option data-identity="3">Bianco </option>
    <option data-identity="4">Max </option>
    <option data-identity="5">Luigi </option>
    <option data-identity="7">Marco </option>
    <option data-identity="8">Verde </option>
</select>