test: remove unused variable

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

PR-URL: https://github.com/nodejs/node/pull/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>