boards: posix: Cleanup backend config for native_posix and nrf52_bsim

Removing backend config from board Kconfig.defconfig and moving
it to native_posix backend configuration in logging. Without this
change define persisted even when logging was not using backends
and that impacted what is compiled in.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>