boards nrf54l15bsim: Remove out of date notes in doc

Remove two out of date/incorrect notes in the documentation for this
board:
For quite a while now, we have models of some parts of the CRACEN HW.
Since f4cdb0f0 we are using the CRACEN
RNG driver.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>