Remove safe-browsing-store unit test helper.

This file originally existed to allow the old sqlite store to share
tests with the new file store.  The old store isn't coming back, nor is
it obvious that future store alternatives will use the same interface.

BUG=none

Review URL: https://codereview.chromium.org/197283011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257027 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 257027