Remove SSL severity score histograms
* The SSL severity scores ended up not yielding promising results, so this CL
removes them.
* This does not remove the histograms that track which errors were caused
by specific causes.
* The "recently expired" heuristic is now being recorded as part of
SSLInterstitialCause instead of as part of a severity score.
* The type of network connection is now being recorded as its own histogram
instead of as part of a severity score.
BUG=
R=palmer@chromium.org
Review URL: https://codereview.chromium.org/
850093003
Cr-Commit-Position: refs/heads/master@{#311636}