This mechanism is incompatible with deploying draft versions of TLS 1.3.

Suppose a draft M client talks to a draft N server, M != N. (Either M or
N could also be the final standard revision should there be lingering
draft clients or servers.) The server will notice the mismatch and
pretend ClientHello.version is TLS 1.2, not TLS 1.3. But this will
trigger anti-downgrade signal and cause an interop failure! And if it
doesn't trigger, all the clever tricks around ServerHello.random being
signed in TLS 1.2 are moot.

We'll put this back when the dust has settled.

Change-Id: Ic3cf72b7c31ba91e5cca0cfd7a3fca830c493a43
Reviewed-on: https://boringssl-review.googlesource.com/11005
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>