This removes the logic in HttpNetworkTransaction to drive the TLS version
fallback. It's gone now.

Note this does come with a behavior change. TLS 1.2 intolerant servers will now
fail with their true error code rather than
ERR_SSL_FALLBACK_BEYOND_MINIMUM_VERSION. That error code was intended for
diagnositics. With the fallback having been gone for several releases (by the
time this hits stable), we won't need it anymore.

Of course, this will all come back for TLS 1.3, but let's implement it in
SSLConnectJob next time. Avoid a bit of munging across layers.

BUG=621780
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2099673003
Cr-Commit-Position: refs/heads/master@{#402312}