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/generic/misc.h

@@ -12,7 +12,6 @@ void console_push_output(uint8_t len);
 uint32_t timer_from_us(uint32_t us);
 void timer_periodic(void);
 uint32_t timer_read_time(void);
-uint8_t timer_set_next(uint32_t next);
 uint8_t timer_try_set_next(uint32_t next);
 
 size_t alloc_maxsize(size_t reqsize);