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This pref was only used for legacy supervised users, based on
supervised_users::kRecordHistoryIncludesSessionSync. Legacy supervised
users don't exist anymore, so no need to keep this around.

Bug: none
Change-Id: I34a3661bba4e9ebc4e89677749f4c0ee001d908d
Reviewed-on: https://chromium-review.googlesource.com/c/1314492
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606007}