nds32: remove unneeded code in arch/nds32/Makefile

 - scripts/Kbuild.include already defined 'comma'

 - The top Makefile has 'PHONY += FORCE'

 - include/asm-*/ was moved to arch/*/include/asm/ a decade ago

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>