m68k/m68knommu: Remove dead SMP config option

CONFIG_SMP doesn't exist in Kconfig (for this architecure), therefore
remove all references to it from the source.

Signed-off-by: Christian Dietrich <qy03fugy@stud.informatik.uni-erlangen.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>