drivers: nrf_rtc_timer: Remove unnecessary locking

As per description of the sys_clock_elapsed() function, "the kernel
will call this with appropriate locking, the driver needs only provide
an instantaneous answer". Remove then the unnecessary locking from the
function, as it only adds an undesirable delay.

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