core/sys/rtimer.[ch]: removed. They're now in the contiki shadow repo
cpu/freertos/rtimer-arch.c (rtimer_arch_schedule): don't run rtimer_run_next

If the delta time is zero or even negative, we just ignore the request
and don't call rtimer_run_next. The new rtimer code takes care of
making sure the timer handler is invoked regardless.
update contiki submodule revision