watchdog: remove stale ifndef CONFIG_WATCHDOG_TIMEOUT_MSECS from wdt.h

Since WATCHDOG_TIMEOUT_MSECS was converted to Kconfig (commit
ca51ef7c), CONFIG_WATCHDOG_TIMEOUT_MSECS has been guaranteed to be
defined. So remove the dead fallback ifdeffery.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>