test: remove harmony flags

v8 6.8 supports all removed flags. For example for BigInt.

PR-URL: https://github.com/nodejs/node/pull/22285


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>