ATXMega ADC - What is bandgap reference and temperature?
The bandgap reference provides an internal reference voltage of 1.1 V; you can then use it by setting REFSEL to 000. You don't need that, you want to use an external reference voltage, i.e. REFSEL 010 or 011. It doesn't matter whether you  enable it or not – as you're not using it. Turning on the bandgap will consume a little power, but typically not significantly much.
Many microcontrollers have a temperature sensor built-in. Bit 0 enables the one in your microcontroller. You can then use the ADC with a special ADC channel to measure what that sensor says. Unless you plan to use that sensor, it doesn't matter whether you turn it on. Again, turning it on will use a  little power.