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>

init/Kconfig

@@ -104,29 +104,9 @@ config COMPILE_TEST
 	  here. If you are a user/distributor, say N here to exclude useless
 	  drivers to be distributed.
 
-config HEADER_TEST
-	bool "Compile test headers that should be standalone compilable"
-	help
-	  Compile test headers listed in header-test-y target to ensure they are
-	  self-contained, i.e. compilable as standalone units.
-
-	  If you are a developer or tester and want to ensure the requested
-	  headers are self-contained, say Y here. Otherwise, choose N.
-
-config KERNEL_HEADER_TEST
-	bool "Compile test kernel headers"
-	depends on HEADER_TEST
-	help
-	  Headers in include/ are used to build external moduls.
-	  Compile test them to ensure they are self-contained, i.e.
-	  compilable as standalone units.
-
-	  If you are a developer or tester and want to ensure the headers
-	  in include/ are self-contained, say Y here. Otherwise, choose N.
-
 config UAPI_HEADER_TEST
 	bool "Compile test UAPI headers"
-	depends on HEADER_TEST && HEADERS_INSTALL && CC_CAN_LINK
+	depends on HEADERS_INSTALL && CC_CAN_LINK
 	help
 	  Compile test headers exported to user-space to ensure they are
 	  self-contained, i.e. compilable as standalone units.