MDEV-22816 Assertion `node->space == fil_system.sys_space' failed in fil_aio_callback

fil_aio_callback(): Remove a bogus assertion that was added in
commit b1ab211d (MDEV-15053).
We will have !bpage for any write by buf_flush_freed_page().

storage/innobase/fil/fil0fil.cc

@@ -3966,10 +3966,9 @@ void fil_aio_callback(os_aio_userdata_t *data)
   buf_page_t *bpage= static_cast<buf_page_t*>(data->message);
   if (!bpage)
   {
-    /* Asynchronous single page writes from the doublewrite buffer
-    don't have access to the page. */
+    /* Asynchronous single page writes from the doublewrite buffer,
+    or calls from buf_flush_freed_page() don't have access to the page. */
     ut_ad(data->type.is_write());
-    ut_ad(node->space == fil_system.sys_space);
     ut_ad(!srv_read_only_mode);
 write_completed:
     mutex_enter(&fil_system.mutex);