build: don't pass `--mode` argument to V8 test-runner

V8's test-runner dropped the `--mode` argument some time back, and now
produces the following error if run with it:
  run-tests.py: error: no such option: --mode

PR-URL: https://github.com/nodejs/node/pull/39055
Refs: https://github.com/nodejs/node/pull/35705


Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>