power: supply: axp20x_usb_power: Remove unused device_node

This member of struct axp20x_usb_power is not used anywhere.
Remove it.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>