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

As far as I know, this test hasn't failed since around the time it was
first reported in 2019.

Closes: https://github.com/nodejs/node/issues/30847

PR-URL: https://github.com/nodejs/node/pull/37462
Fixes: https://github.com/nodejs/node/issues/30847


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>