<script>
function foo() {
    var aTR = document.getElementsByClassName("ms-cal-gempty")[0].getElementsByTagName("tr"),
    elmParent = aTR[0].parentNode, sHtml;

    for (var i=aTR.length-1; i>=0; i--) {
        sHtml = aTR[i].innerHTML;
        if (sHtml.toLowerCase().indexOf("td")>-1 && sHtml.indexOf("ms-cal-walldayevent")<0) {
            elmParent.removeChild(aTR[i]);
        }
    }
}
</script>
<button onclick="foo">Fjern linjer</button>