kbuild: remove unused archmrproper

No one uses archmrproper.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index 499b968..ff1054c 100644
--- a/Makefile
+++ b/Makefile
@@ -1360,11 +1360,11 @@
 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)