backend: remove debug du thread exit notice

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/backend.c b/backend.c
index dd7dc67..14803c3 100644
--- a/backend.c
+++ b/backend.c
@@ -2009,10 +2009,8 @@
 		}
 
 		ret = pthread_cond_timedwait(&du_cond, &du_lock, &ts);
-		if (ret != ETIMEDOUT) {
-			printf("disk thread should exit %d\n", ret);
+		if (ret != ETIMEDOUT)
 			break;
-		}
 
 		ret = update_io_ticks();