um: clean up archheaders recipe

Now that '%asm-generic' is added to no-dot-config-targets,
'make asm-generic' does not include the kernel configuration.

You can simply do 'make asm-generic' in the recursed top Makefile
without bothering syncconfig.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Richard Weinberger <richard@nod.at>

arch/um/Makefile

@@ -116,13 +116,7 @@ endef
 KBUILD_KCONFIG := $(HOST_DIR)/um/Kconfig
 
 archheaders:
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
-	            kbuild-file=$(HOST_DIR)/include/asm/Kbuild \
-		    obj=$(HOST_DIR)/include/generated/asm
-	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
-	            kbuild-file=$(HOST_DIR)/include/uapi/asm/Kbuild \
-		    obj=$(HOST_DIR)/include/generated/uapi/asm
-	$(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) archheaders
+	$(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) asm-generic archheaders
 
 archprepare: include/generated/user_constants.h