test: remove flaky designation for test

Unreliability for test-trace-events-fs-sync is believed to have been
fixed. Remove flaky designation.

Ref: https://github.com/nodejs/node/pull/22812
Ref: https://github.com/nodejs/node/issues/21038#issuecomment-421199960

PR-URL: https://github.com/nodejs/node/pull/22856
Refs: https://github.com/nodejs/node/pull/22812


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Refael Ackermann <refack@gmail.com>