kbuild: remove unused archmrproper

No one uses archmrproper.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1360,11 +1360,11 @@ mrproper: rm-dirs  := $(wildcard $(MRPROPER_DIRS))
 mrproper: rm-files := $(wildcard $(MRPROPER_FILES))
 mrproper-dirs      := $(addprefix _mrproper_,scripts)
 
-PHONY += $(mrproper-dirs) mrproper archmrproper
+PHONY += $(mrproper-dirs) mrproper
 $(mrproper-dirs):
 	$(Q)$(MAKE) $(clean)=$(patsubst _mrproper_%,%,$@)
 
-mrproper: clean archmrproper $(mrproper-dirs)
+mrproper: clean $(mrproper-dirs)
 	$(call cmd,rmdirs)
 	$(call cmd,rmfiles)
 

arch/h8300/Makefile

@@ -37,8 +37,6 @@ libs-y	+= arch/$(ARCH)/lib/
 
 boot := arch/h8300/boot
 
-archmrproper:
-
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)