documentation: watchdog: remove documentation of at32ap700x_wdt

Since at32ap700x_wdt is gone, no need to keep its documentation

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>