drivers: nrf_rtc_timer: Remove unnecessary interrupt locking

There is no need to disable interrupts while just checking if
a channel needs to be processed in the ISR, as that section
does not contain anything that needs to be protected against
overwriting from some other context. In particular, if a given
timeout is changed or even aborted while its event is being
checked, this will be correctly handled in the code that follows
and that checks the expiration time.

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