scripts/tests/twister_blackbox: Don't test on native_posix

native_posix is now deprecated.
Building this sample in both native_sim and native_posix does not
improve coverage for the sample or subsystem but doubles CI time.
As anyhow native_posix will be removed all together in
2 releases, let's remove it already for this sample.

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