asm-generic/system.h: drop useless __KERNEL__

This header isn't exported to user-space, and even if it was, the
__KERNEL__ check covers the entire file, so we'd get a useless stub in the
first place.  So punt it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>