drivers: timer: nrf: remove unnecessary event feature

Enabling the RTC event is intended to support peripheral-to-peripheral
interconnects, so introduces a request for HFCLK and PCLK16M when the
event is triggered.  This specific event is never used with PPI so
enabling events apparently does nothing but increase power consumption.

Closes #15513

Signed-off-by: Peter A. Bigot <pab@pabigot.com>