I assume you are referring to the 5V for USB_VBUS? If you are not using the interface you should not have to provide this voltage, although I haven't tested this myself. USB VBUS is only used to determine operating mode. The other supplies for the USB device are shared on the bank voltages so those would be needed even if you didn't use USB. To be on the safe side you could always hold the device in reset using the P1_USB0_RST_N signal, that way the device is tristated anyway. I'm a little out of my zone here, but I believe you can drive that signal from the I/O expander using the I2C bus
