Fix use-after-free in HDDDTC shutdown
We were trying to unregister the HistoryDeleteDirective DTC at shutdown, but
the DTC shutdown happens as part of the ProfileSyncService shutdown, and is
destroyed after the observers_ object is destroyed. As such, there's no need
to remove the observer.
This patch also unreverts
6513171d7473b9eb60e1bf16369cf893daa33a07
Original codereview at https://codereview.chromium.org/
534733002
BUG=409965 368834
Review URL: https://codereview.chromium.org/
535683002
Cr-Commit-Position: refs/heads/master@{#293824}