nrf52_bsim: Fix unfreed memory

There was a chunk of unfreed memory left over at exit().
Clear it as it should be.
The issue is only a cause for a warning in valgrind,
as Linux frees all process booked memory on exit,
but nonetheless it is better fixing it.

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