test: remove second arg from assert.ifError()

`test/parallel/test-fs-readfile.js` has a call to
`assert.ifError()` that receives two arguments.

There is no second argument used in `assert.ifError()`.
This PR removes this argument.

PR-URL: https://github.com/nodejs/node/pull/22190


Reviewed-By: George Adams <george.adams@uk.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>