thread-pool: drop thread_pool_active()

.io_flush() is no longer called so drop thread_pool_active().  The block
layer is the only thread-pool.c user and it already tracks in-flight
requests, therefore we do not need thread_pool_active().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

thread-pool.c

diff --git a/thread-pool.c b/thread-pool.c
index 0ebd4c2..096f007 100644
--- a/thread-pool.c
+++ b/thread-pool.c
@@ -197,12 +197,6 @@
     }
 }
 
-static int thread_pool_active(EventNotifier *notifier)
-{
-    ThreadPool *pool = container_of(notifier, ThreadPool, notifier);
-    return !QLIST_EMPTY(&pool->head);
-}
-
 static void thread_pool_cancel(BlockDriverAIOCB *acb)
 {
     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
@@ -310,7 +304,7 @@
     QTAILQ_INIT(&pool->request_list);
 
     aio_set_event_notifier(ctx, &pool->notifier, event_notifier_ready,
-                           thread_pool_active);
+                           NULL);
 }
 
 ThreadPool *thread_pool_new(AioContext *ctx)