posix-timers: Use timer_rearm() callback in posixtimer_rearm()

Use the new timer_rearm() callback to replace the conditional hardcoded
calls into the hrtimer and cpu timer code.

This allows later to bring the same logic to alarmtimers.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211656.889661919@linutronix.de

kernel/time/posix-timers.h

@@ -28,5 +28,3 @@ extern const struct k_clock clock_thread;
 extern const struct k_clock alarm_clock;
 
 int posix_timer_event(struct k_itimer *timr, int si_private);
-
-void posix_cpu_timer_schedule(struct k_itimer *timer);