kbuild: remove stale dependency between Documentation/ and headers_install

Commit 8e2faea8 ("Make Documenation depend on headers_install")
dates back to 2014, which is before Sphinx was introduced for the
kernel documentation.

Since none of DOC_TARGET requires headers_install, it is strange to
run it only for the single target "Documentation/".
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1744,8 +1744,7 @@ build-dir = $(patsubst %/,%,$(dir $(build-target)))
 PHONY += /
 /: ./
 
-# Make sure the latest headers are built for Documentation
-Documentation/ samples/: headers_install
+samples/: headers_install
 %/: prepare FORCE
 	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir)