Revert of Check to ensure PowerObservers are added and removed on the same thread...

Revert of Check to ensure PowerObservers are added and removed on the same thread (patchset #7 id:120001 of https://codereview.chromium.org/502003003/)

Reason for revert:
Causing failures on Mac try jobs:
ExtensionServiceTest.ExternalExtensionAutoAcknowledgement (run #1):
[ RUN      ] ExtensionServiceTest.ExternalExtensionAutoAcknowledgement
[8427:1799:0904/211500:4724222818698:FATAL:power_monitor.cc(38)] Check failed: obs->power_monitor_thread_checker_.

Original issue's description:
> Check to ensure PowerObservers are added and removed on the same thread
>
> This behavior is required by ObserverListThreadSafe and may leads to crashes if not followed
>
> BUG=404767
>
> Committed: https://chromium.googlesource.com/chromium/src/+/518c46fbba70c91169bfc0c64b542f06e56b3915

TBR=thakis@chromium.org,brettw@chromium.org,scottmg@chromium.org,bajones@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=404767

Review URL: https://codereview.chromium.org/544913003

Cr-Commit-Position: refs/heads/master@{#293448}