test: remove unused param in test-graph.pipe

Refs: https://twitter.com/NodeTodo/status/900502354834800645

PR-URL: https://github.com/nodejs/node/pull/15007
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/async-hooks/test-graph.pipe.js

@@ -14,7 +14,7 @@ sleep
   .on('exit', common.mustCall(onsleepExit))
   .on('close', common.mustCall(onsleepClose));
 
-function onsleepExit(code) {}
+function onsleepExit() {}
 
 function onsleepClose() {}