config["sysfreq"] should not be reset on refresh

[cboulay]

I'm not sure of the cause, maybe REQCONFIGALL doesn't always return SYSREPRUNLEV, but refreshing the config sometimes results in config["sysfreq"] being None.

As this isn't something that the device can change, there's really no need to reset this variable.

• Initialize config["sysfreq"] = None somewhere else only once
• Remove this line from the config refresh method