Revert "net/http: fix data race due to writeLoop goroutine left running"

This reverts CL 232799.

Reason for revert: net/http test is failing on all longtest builders.

Change-Id: I4694e34f35419bab2d0b45fa6d8c3ac2aa1f51a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/250597


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>