samples/net: Don't test on native_posix

native_posix is now deprecated.
Building these samples in both native_sim and native_posix
does not improve coverage for these samples 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>