Skip initial roughtime result when EnableRoughtime is disabled

[terencechain]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?
Even when EnableRoughtime was disabled, it still runs through the following code:

	// Log Debug Results.
	for _, res := range results {
		if res.Error() != nil {
			log.Errorf("Could not get rough time result: %v", res.Error())
			continue
		}
		log.WithFields(logrus.Fields{
			"Server Name": res.Server.Name,
			"Midpoint":    res.Midpoint,
			"Delay":       res.Delay,
			"Radius":      res.Roughtime.Radius,
		}).Debug("Response received from roughtime server")
	}

This PR makes it exit earlier when EnableRoughtime is false in function recalibrateRoughtime()

Which issues(s) does this PR fix?

Fixes #7183

Other notes for review