hardirq.h: remove now-empty #ifdef/#endif pair

Commit 451a3c24b013 ("BKL: remove extraneous #include <smp_lock.h>")
removed the #include line that was the only thing that was surrounded by
the #ifdef/#endif.

So now that #ifdef is guarding nothing at all. Just remove it.

Reported-by: Byeong-ryeol Kim <brofkims@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>