Remove unused decision callback from SSL blocking pages.

With committed interstitials, this callback is no longer used.

Also remove OnProceed and OnDontProceed from some SSL pages (which
have been superseded by handling in SSLErrorControllerClient), and
DCHECK that the blocking page ready callback is non-null.

Bug: none
Change-Id: Ibbc7f07827ec9c2bd68bae5a7505bd44a9bf94f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888743
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711517}