ARM: dts: Remove virtgpio from bcm2711-rpi-4-b.dts

Pi 4B doesn't need the virtgpio driver (it was added to reduce the
overhead of driving the ACT LED on the Pi 3B's GPIO expander), so
remove the instantiation by the Pi 4 DTS file to avoid confusion.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>