reverseproxy: Remove redundant flushing

[francislavoie]

From reading through the code, I think this code path is now obsoleted by the changes made in #4266.

Basically, h.flushInterval() will set the flush interval to -1 if we're in a bi-directional stream, and the recent PR ensured that h.copyResponse() properly flushes headers immediately when the flush interval is non-zero. So now there should be no need to call Flush before calling h.copyResponse().

---

Since this was your code from #3556 and #3561 @masknu, would you mind confirming that your usecase still works correctly with this change, just as a sanity check? Thanks!

[francislavoie]

Well, tests are passing, and there was a test added along with the original change (

caddy/caddytest/integration/stream_test.go

Lines 22 to 23 in ce5a45d

	// (see https://github.com/caddyserver/caddy/issues/3556 for use case)
	func TestH2ToH2CStream(t *testing.T) {

), so I think we're good to go here, so we can merge unless we hear back in the next couple days.

[mholt]

Ok, good point. I'll merge this, and we can revert if we find out otherwise.