test: remove unused param in test-graph.pipe

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

PR-URL: #15007
Reviewed-By: Yuta Hiroto <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: ssbrewster

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() {}