tests: remove testing for board unit_testing

Remove boilerplate code:
> if (NOT BOARD STREQUAL unit_testing)
>    message(FATAL_ERROR "This project can only be used with...")
> endif()

as that is now handled by boards.cmake when validating the board.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>