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 <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Alexey Orlenko <[email protected]>

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();
     }));