kbuild: Makefile: remove unnecessary check for m68knommu ARCH

ARCH is never set to m68knomm.
make ARCH=m68knomm is not supported anymore.

Signed-off-by: Edward Shao <laface.tw@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>