sched: Avoid rescheduling the currently active timer

It's tricky to reschedule the timer irq correctly (due to race
conditions with the irq) and in practice it's very rarely needed.
Handle the special cases in the generic sched.c code so that the board
code doesn't have to handle it.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/simulator/main.c

@@ -69,12 +69,6 @@ timer_read_time(void)
     return 0; // XXX
 }
 
-uint8_t
-timer_set_next(uint32_t next)
-{
-    return 0;
-}
-
 uint8_t
 timer_try_set_next(uint32_t next)
 {