function SelectedComboItems()
{
  var selItem = document.getElementById('ddl');
  var i;
  for (i = selItem.length - 1; i>=0; i--) 
  {
    if (selItem.options[i].selected) 
    {
      selItem.remove(i);
    }
  }
}

Add the above javascript function button Attributes in Code behind file.

btnRemove.Attributes.Add("onclick","SelectedComboItems();")