Remove safe-browsing code to fix up injected prefixes.

Remove safe-browsing code to fix up injected prefixes.

An older implementation of the safe-browsing database code injected
prefixes when receiving full hashes, due to an incorrect reading of how
full-hash matches should happen.  Current code checks the full hashes
and prefixes independently, so the duplicate prefixes are no longer
generated.  This code removed duplicate prefixes found in existing
databases.  99.99% of runs over the past two weeks have not removed any
such duplicate prefixes.

In the current code hitting a full hash has the same end result as
hitting a prefix, so the primary downside of removing this code is
slight excess storage.  Insofar as full hashes are deleted by sub, there
may be stale prefixes left behind.  The stale prefixes will be deleted
when the chunk in question is deleted.  The result may be a small number
of unexpected gethash requests which will return misses (and thus not
affect the browsing experience other than adding a little latency).

BUG=361248
Committed: https://crrev.com/6434e337b1c0eba3c30d497d7d1a785f4a3d1e5b
Cr-Commit-Position: refs/heads/master@{#292019}

[Scott Hess - ex-Googler, 2014-08-26 19:40:08]

shess@chromium.org changed reviewers:
+ asvitkine@chromium.org, mattm@chromium.org

[Scott Hess - ex-Googler, 2014-08-26 19:40:08]

Original CL this mostly reverts:
  https://codereview.chromium.org/263833005/

asvitkine, histograms.xml change just marks something obsolete.

mattm, let me know if you think the CL comment is right, too.  I can't think of
any false-negative cases, just some false-positive cases, which seems reasonable
at these volumes.  I honestly suspect that the databases which haven't already
been stripped are having update issues or something.

[Alexei Svitkine (slow), 2014-08-26 20:25:57]

lgtm, but only looked at the xml

[mattm, 2014-08-26 20:49:46]

lgtm

[Scott Hess - ex-Googler, 2014-08-26 20:50:45]

The CQ bit was checked by shess@chromium.org

[commit-bot: I haz the power, 2014-08-26 20:51:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/shess@chromium.org/507653003/20001

[commit-bot: I haz the power, 2014-08-26 22:20:41]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_chromeos_rel_swarming on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[commit-bot: I haz the power, 2014-08-26 23:04:02]

Committed patchset #2 (20001) as 8e3ab2c41aab3edb6e046ba4ed8036f072b31458

[commit-bot: I haz the power, 2014-09-10 02:46:58]

Patchset 2 (id:??) landed as
https://crrev.com/6434e337b1c0eba3c30d497d7d1a785f4a3d1e5b
Cr-Commit-Position: refs/heads/master@{#292019}