rcupdate: remove dead code

DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT, so #ifndef CONFIG_PREEMPT
is totally useless in kernel/rcupdate.c.

Signed-off-by: WANG Cong <[e-mail]>
Cc: Paul E. McKenney <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

kernel/rcupdate.c

···
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
 		 */
-#ifndef CONFIG_PREEMPT
-		WARN_ON(1);
-		return 0;
-#else
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON(1);
···
 		rcu_barrier_bh();
 		debug_object_free(head, &rcuhead_debug_descr);
 		return 1;
-#endif
 	default:
 		return 0;
 	}