Remove the alternative preferences filename from JsonPrefStore.

The alternative pref filename kProtectedPreferencesFilenameDeprecated
was slated for removal sometime after M40. This removes it and the code
supporting it in JsonPrefStore.

BUG=654988

Review-Url: https://codereview.chromium.org/2751603002
Cr-Commit-Position: refs/heads/master@{#457310}
7 files changed