Revert "io: detect Writers that access io.Copy's buffer after returning"

Revert "io: detect Writers that access io.Copy's buffer after returning"

This reverts CL 466865.

Reason for revert: Causing trybot flakiness due to net/http race,
roll back until net/http is fixed.

For #58168

Change-Id: I3129deb996abe6466eccf933fe93cbbaf72ae217
Reviewed-on: https://go-review.googlesource.com/c/go/+/467895
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>