The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2b8d04e8de49d942d5d9f5d230e66278e873187d

commit 2b8d04e8de49d942d5d9f5d230e66278e873187d
Author: Varun Khaneja <vakh@chromium.org>
Date: Thu Jan 03 23:51:37 2019

Remove DeletePVer3StoreFiles since it is no longer required.

PVer4 rolled out in M57.

I had added code to delete PVer3's files from disk along with a comment
saying:
// This method can be removed after the UMA metrics for the following prefix
// go down to 0 in Stable: "SafeBrowsing.V4UnusedStoreFileExists.V3."

Well [1][2] shows that this metric is down to 0 for all stores now.
[1]: https://uma.googleplex.com/p/chrome/timeline_v2/?sid=6ebed968120f27fb42f41b6a21afb541
[2]: http://screen/NWvJt0wwNjv

R=drubery

Bug:  916192 
Change-Id: I504c51c32a1247aa3375c13c902ec12612c46429
Reviewed-on: https://chromium-review.googlesource.com/c/1382896
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619815}
[modify] https://crrev.com/2b8d04e8de49d942d5d9f5d230e66278e873187d/components/safe_browsing/db/v4_local_database_manager.cc
[modify] https://crrev.com/2b8d04e8de49d942d5d9f5d230e66278e873187d/components/safe_browsing/db/v4_local_database_manager.h
[modify] https://crrev.com/2b8d04e8de49d942d5d9f5d230e66278e873187d/tools/metrics/histograms/histograms.xml