test: remove invalid test

`test/known_issues/test-stdout-buffer-flush-on-exit.js` is invalid. The
behavior seen currently (with the test failing) is in accordance with
documentation which indicates that calling `process.exit()` may mean
scheduled asynchronous I/O does not happen. The documentation also
indicates that `process.stdout` is asynchronous when it is a POSIX pipe.

PR-URL: #15320
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>