watchdog: drop redundant 'default n' option for WATCHDOG_SYSFS

The option 'default n' and its absence are equal for kbuild,
which makes explicit 'default n' redundant.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.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>