net/http: deflake request-not-written path

When we receive an error writing the first byte of a request to a
reused connection, we retry the request on a new connection. Remove
a flaky path which could cause the request to not be retried if
persistConn.roundTrip reads the error caused by closing the connection
before it reads the write error that caused the connection to be
closed.

Fixes #30938.

Change-Id: Iafd99e3239cd9dba4a4c9ddd950a877ca9815e59
Reviewed-on: https://go-review.googlesource.com/c/go/+/379554


Trust: Bryan Mills <bcmills@google.com>
Trust: Damien Neil <dneil@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>