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: Masahiro Yamada <yamada.masahiro@socionext.com>