driver: nrf_rtc_timer: Remove unnecessary setting of comparator

Remove a piece of code that was supposed to bring an extra update
of the anchor value but which in fact was not able to provide it,
because of the target time checking performed in process_channel(),
and which is anyway unnecessary because the timeout span is limited
to MAX_CYCLES in sys_clock_set_timeout(), so the timeout handler is
guaranteed to be executed at least twice per each RTC overflow.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>