test: refactor test-fs-readfile-tostring-fail

The test uses both `assert()` and `assert.ok()`. Use just `assert.ok()`.

Remove a comment that does not appear to match the code and does not
seem to explain much beyond the bare code itself.

PR-URL: https://github.com/nodejs/node/pull/19404


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>