Revert "Make if_timer MP-safe if IFEF_MPSAFE"

Because it has decreased the performance of wm. And also I found that
wm_watchdog doesn't work well with if_watchdog framework at all. Sharing one
counter (if_timer) with multiple instances (hardware multi-queues) can't detect
a single (or some) stall of them because other instances reset the counter even
if the stalled one want the watchdog to fire.

Interfaces without IFEF_MPSAFE works safely with the original if_watchdog thanks
to KENREL_LOCK. OTOH, interfaces with IFEF_MPSAFE shouldn't use if_watchdog and
should implement their own watchdog timer that works with multiple instances.

(ozaki-r)