[policy] Remove observers log in CloudPolicyService

The log line is never shown in logs and does not help debugging the
crash.Underlying crash line in ~ObserverList() can log remaining
observers itself.

Bug: b:40915114
Change-Id: I7c283e11697e1ed8af29f3b73dec3bdb5306e38c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6181816
Commit-Queue: Artem Sumaneev <asumaneev@google.com>
Reviewed-by: Owen Min <zmin@chromium.org>
Auto-Submit: Artem Sumaneev <asumaneev@google.com>
Reviewed-by: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1409563}