This is what I used, which appears to be working without issues. Just added 2 jQuery lines to the JavaScript function. These lines set the css display attribute to none for all elements that have SortAsc or SortDesc in their id.
function RemoveSorting() {
var tableView = $find("<%= RadGrid1.ClientID %>").get_masterTableView();
//clear sorting on all data columns ( but not the asc/desc images)
tableView.get_sortExpressions().clear();
//make all asc images invisible
$("[id*=SortAsc]").each(function (index) { $(this).css('display', 'none'); });
//make all desc images invisible
$("[id*=SortDesc]").each(function (index) { $(this).css('display', 'none'); });
tableView.rebind();
}