Unwind fallback metrics and SSLFailureState.

Part 2 of removing the TLS version fallback code.

These were of great help, but now that the fallback is dead and gone, we don't
need them anymore. (Until it's time to bring back the fallback for TLS 1.3. :(
But next time let's not require quite so much glue in net/http.)


Review-Url: https://codereview.chromium.org/2093873002
Cr-Commit-Position: refs/heads/master@{#401917}
28 files changed