gpio: axp/sunxi: Remove virtual VBUS detection GPIO

Now that this functionality is modeled using the device tree and
regulator uclass, the named GPIO is not referenced anywhere. Remove it.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>