power: reset: gpio-restart: Convert to platform remove callback returning void

Returning an error code from a remove callback makes the driver core
emit a generic (and so little helpful) error message and after that the
error code is ignored and the device unbound.

As gpio_restart_remove() already emits an error message, suppressing the
generic error is no loss.

Then convert to .remove_new() which is equivalent to returning 0
unconditionally in .remove(). See commit 5c5a7680 ("platform:
Provide a remove callback that returns no value") for its rational.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>