Make CreateShortcut operation a NOOP on non-Windows systems

The primary (only?) use case of the 'CreateShortcut' operation is on Windows. However, the operation is still executed on other systems, which forces users to write

    if (installer.value("os") === "win") {
        component.addOperation("CreateShortcut", installer.value("TargetDir") + "/README.txt", installer.value(StartMenuDir") + "/README.lnk");
    }

. We should just making the operation a NOOP on other systems.