Revert "boards: nordic: nRF54L15DK: set HFXO clock latency to 854"

This reverts commit d0a07698.

The reason for reverting this is that the only safe startup time for
54L series is the one defined in the SoC .dtsi files (1650us), and other
startup times are not reliable even when tested on a particular board.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>