watchdog: rn5t618: Simplify using devm_watchdog_register_device()

watchdog: rn5t618: Simplify using devm_watchdog_register_device()

This allows to drop the .remove() function as it only exists to
unregister the watchdog device which is now done in a callback
registered by devm_watchdog_register_device().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230307070404.2256308-3-u.kleine-koenig@pengutronix.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>