Remove DarkModeObserver

As a result of the following changes, DarkModeObserver is no longer
in use and can be removed:
https://crrev.com/c/1671467
https://crrev.com/c/1670052
https://crrev.com/c/1684132/13..14
https://crrev.com/c/1666437

Bug: 970285
Change-Id: I1fc397757a0cfd387c1bfa3cfcd4d353aacda2e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689910
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Alison Maher <almaher@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#677534}
6 files changed