What to do with USB power and ground when you have an external power supply?
You could leave it disconnected but it is conventional practice to do two things with the VBus Line when you are an externally powered device.


VBus is often connected into a detection circuit that will determine if 5V is present or not. This is useful for the device to know that it is connected to the host or not.
USB D+ and D- lines are often equipped with clamp diode packages to help eliminate transient spikes that may come in the cable of the USB. It is sometimes appropriate to connect these clamp devices between the USB GND and the USB VBUS.