This reverts commit f73e4c9e.

Reason for revert: This turned out to cause more churn and provide less
security than expected.

Updates golang/go#24513

Change-Id: I2c8d0c39f8759ec8895a3261c91a98aeb2303ede
Reviewed-on: https://go-review.googlesource.com/117955
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>