documentation: watchdog: remove documentation of w83697hf_wdt/w83697ug_wdt

Since w83697hf_wdt/w83697ug_wdt watchdogs drivers were removed
in commit 7285fae9 ("watchdog: Remove drivers for W83697HF and W83697UG")
There are no need to keep their 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>