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

Commit 451a3c24 ("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>

include/linux/hardirq.h

@@ -2,8 +2,6 @@
 #define LINUX_HARDIRQ_H
 
 #include <linux/preempt.h>
-#ifdef CONFIG_PREEMPT
-#endif
 #include <linux/lockdep.h>
 #include <linux/ftrace_irq.h>
 #include <asm/hardirq.h>