m68knommu: fix make archclean

Remove build reference to arch/m68knommu/boot directory, it doesn't
exist.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/m68knommu/Makefile

@@ -118,4 +118,4 @@ core-y	+= arch/m68knommu/kernel/ \
 libs-y	+= arch/m68knommu/lib/
 
 archclean:
-	$(Q)$(MAKE) $(clean)=arch/m68knommu/boot
+