CSP2: Drop 'unsafe-redirect'.

Neither Chrome nor Firefox have implemented the `unsafe-redirect` bits of
CSP2 (see 1.2 of http://www.w3.org/TR/CSP2/#changes-from-level-1).
Experimentation locally on internal sites leads me to believe that it's not
going to be web compatible: I didn't find any Google property that used CSP
which the new behavior wouldn't break in some way.

Moreover, statistics from Chrome show that about 0.7% of page views would
be affected (trending upwards), which is significantly above the bar that
Blink has for breaking changes:
https://www.chromestatus.com/metrics/feature/timeline/popularity/709

We didn't declare it as "at risk" in the CSP2 CR, though we probably should
have. I think this means that removing the source expression and associated
behavior is likely a "substantive change"[1], which I think means that we
need to revise and republish the CR. Wendy, can you confirm that?

(Note to self: next time, mark the whole spec as "at risk" to make
deletions take less than a month and a half. :) )

[1]: http://www.w3.org/2014/Process-20140801/#substantive-change
[2]: http://www.w3.org/2014/Process-20140801/#revised-cr

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Mike West <mkwst@google.com>, @mikewest

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