core/kernel_defines.h: drop BUILD_BUG_ON()
core/kernel_defines.h: drop BUILD_BUG_ON()

This provides the same functionality as `static_assert()` provided by
C11 and has no advantages compared to it. Hence, encourage users to use
standard C functionality instead.

miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT