Remove a reference check

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/ioengines.c b/ioengines.c
index 230e846..c143b33 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -398,8 +398,6 @@
 
 int td_io_close_file(struct thread_data *td, struct fio_file *f)
 {
-	assert(f->references);
-
 	if (!fio_file_closing(f))
 		log_file(td, f, FIO_LOG_CLOSE_FILE);