kbuild: drop check for CONFIG_ in headers_check

The check for references to CONFIG_ symbols in exported headers turned
out to be too agressive with the current state of affairs.
After the work of Jaswinder to clean up all relevant cases we are down
to almost pure noise.

So lets drop the check for now - we can always add it back later
should our headers be ready for that.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>