kbuild: remove header compile test

There are both positive and negative options about this feature.
At first, I thought it was a good idea, but actually Linus stated a
negative opinion (https://lkml.org/lkml/2019/9/29/227). I admit it
is ugly and annoying.

The baseline I'd like to keep is the compile-test of uapi headers.
(Otherwise, kernel developers have no way to ensure the correctness
of the exported headers.)

I will maintain a small build rule in usr/include/Makefile.
Remove the other header test functionality.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

drivers/gpu/drm/i915/Kconfig.debug

@@ -7,7 +7,6 @@ config DRM_I915_WERROR
         # We use the dependency on !COMPILE_TEST to not be enabled in
         # allmodconfig or allyesconfig configurations
         depends on !COMPILE_TEST
-	select HEADER_TEST
         default n
         help
           Add -Werror to the build flags for (and only for) i915.ko.