Refactor: controller: Clean up controld_verify_stonith_watchdog_timeout

pcmkvalid_stonith_watchdog_timeout() already calls crm_get_msec(value)
if value is not NULL. The only thing we lose by skipping the
pcmkvalid_stonith_watchdog_timeout() call for the "st_timeout == 0"
case is a debug message.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>