[release-branch.go1.13] net/http: remove TestTimeoutHandlerAndFlusher due to flakes

Removes TestTimeoutHandlerAndFlusher due to flakes on
one of the builders due to timing issues.

Perhaps later, we might need to bring it back when we've
figured out the timing issues.

Updates #34573
Fixes #34579

Change-Id: Ia88d4da31fb228296144dc31f9a4288167fb4a53
Reviewed-on: https://go-review.googlesource.com/c/go/+/197757


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 55738850)
Reviewed-on: https://go-review.googlesource.com/c/go/+/197719


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>