test: remove FLAKY designation for test-http2-client-upload-reject

The test hasn't failed in a long time as far as I can tell. The issue
may have been fixed by various event updates/changes to http2 and
related systems.

PR-URL: https://github.com/nodejs/node/pull/37461


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>