Avoiding modification of vm->retval in njs_promise_alloc().

Alloc functions are not expected to modify existing values.
diff --git a/src/njs_promise.c b/src/njs_promise.c
index c676f68..a31f2cc 100644
--- a/src/njs_promise.c
+++ b/src/njs_promise.c
@@ -107,7 +107,6 @@
     njs_queue_init(&data->fulfill_queue);
     njs_queue_init(&data->reject_queue);
 
-    njs_set_promise(&vm->retval, promise);
     njs_set_data(&promise->value, data, 0);
 
     return promise;