samples: shell: Fix filtering for POSIX arch

One of these samples was filtering all POSIX arch
boards out by explictly listing them by name
(and for the nrf52_bsim by lack of UART).

Instead filter by the architecture.

There is no functional difference in tree with this change.

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