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>