Get rid of use of reflection to call Thread.isVirtual() in nsk/jdi/EventRequestManager/stepRequests/stepreq001t.java

The following code is in nsk/jdi/EventRequestManager/stepRequests/stepreq001t.java:

    private static boolean isVirtual(Thread thread) {
        try {
            Method isVirtual = Thread.class.getMethod("isVirtual");
            return (boolean) isVirtual.invoke(thread);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

I think this is a relic from the early days of loom when Thread.isVirtual() was not yet a public method. It can be removed and the callers can call Thread.isVirtual() directly.