macOS 10.13 check in TabButtonAccessibility.m can be removed

After JDK-8317970 bumps the minimum macOS version for the JDK to macOS 11 we can remove the following check
- (NSAccessibilitySubrole)accessibilitySubrole
{
    if (@available(macOS 10.13, *)) {
        return NSAccessibilityTabButtonSubrole;
    }
    return NSAccessibilityUnknownSubrole;
}