test: remove vulgar language

The FIXME is obsolete as it was meant about a indentation issue
that got fixed a long time ago.

PR-URL: nodejs#18395
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

test/parallel/test-zerolengthbufferbug.js

@@ -6,7 +6,6 @@ const http = require('http');
 
 const server = http.createServer((req, res) => {
   const buffer = Buffer.alloc(0);
-  // FIXME: WTF gjslint want this?
   res.writeHead(200, { 'Content-Type': 'text/html',
                        'Content-Length': buffer.length });
   res.end(buffer);