soc: xtensa: sample_controller: Cleanup backend config

Removing backend config from board Kconfig.defconfig and moving
it to xtensa_sim 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>