fix(s2n-quic): make AsyncWrite::poll_flush a no-op #4913

Job	Run time
env	11s
s2n-quic-qns (debug)	2m 37s
s2n-quic-qns (release)	7m 2s
bench	18m 27s
attack (udp)	5m 36s
h3spec (s2n-tls)	14s
interop (aioquic, s2n-quic)	15m 39s
perf (s2n-quic, s2n-quic)	1m 25s
h3spec (rustls)	13s
interop (chrome, s2n-quic)	5m 21s
perf (s2n-quic-null, s2n-quic-null)	59s
interop (go-x-net, s2n-quic)	11m 41s
interop (kwik, s2n-quic)	14m 17s
interop (lsquic, s2n-quic)	13m 1s
interop (msquic, s2n-quic)	17m 16s
interop (mvfst, s2n-quic)	11m 55s
interop (neqo, s2n-quic)	15m 10s
interop (ngtcp2, s2n-quic)	14m 22s
interop (picoquic, s2n-quic)	16m 29s
interop (quic-go, s2n-quic)	12m 38s
interop (quiche, s2n-quic)	18m 30s
interop (quinn, s2n-quic)	14m 37s
interop (s2n-quic, aioquic)	13m 22s
interop (s2n-quic, go-x-net)	11m 34s
interop (s2n-quic, haproxy)	15m 49s
interop (s2n-quic, kwik)	12m 31s
interop (s2n-quic, lsquic)	11m 39s
interop (s2n-quic, msquic)	11m 18s
interop (s2n-quic, mvfst)	9m 21s
interop (s2n-quic, neqo)	12m 56s
interop (s2n-quic, nginx)	10m 52s
interop (s2n-quic, ngtcp2)	12m 38s
interop (s2n-quic, picoquic)	12m 2s
interop (s2n-quic, quic-go)	10m 35s
interop (s2n-quic, quiche)	12m 2s
interop (s2n-quic, quinn)	12m 10s
interop (s2n-quic, s2n-quic)	13m 24s
interop (s2n-quic, s2n-quic-rustls)	13m 4s
interop (s2n-quic, xquic)	12m 17s
interop (s2n-quic-rustls, s2n-quic)	12m 9s
interop (xquic, s2n-quic)	19m 55s
perf-report	10s
interop-report	17s
	7h 37m 45s

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