test: remove unused variable

Remove unused variable err in
  test/sequential/test-http-writable-true-after-close.js

PR-URL: #17186
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>

test/sequential/test-http-writable-true-after-close.js

@@ -34,7 +34,7 @@ const server = createServer(common.mustCall((req, res) => {
     }));
   }).listen(0, () => {
     external = get(`http://127.0.0.1:${server.address().port}`);
-    external.on('error', common.mustCall((err) => {
+    external.on('error', common.mustCall(() => {
       server.close();
       internal.close();
     }));