Revert "[messaging] Add change delta observer shell for TabGroupSyncService"

This reverts commit 136ff32af269394f2ca1dfc950596d4724a6300b.

Reason for revert: this causes crashes for multiple tests on multiple bots, for example https://ci.chromium.org/ui/test/chrome/org.chromium.chrome.browser.collaboration.CollaborationServiceFactoryTest%23testServiceCreation_RealService?q=V%3Abuilder%3Dandroid-arm64-tests+V%3Adevice_os%3DR+V%3Adevice_type%3Dflame+V%3Aos%3DAndroid+V%3Atest_suite%3Dchrome_public_test_apk

Original change's description:
> [messaging] Add change delta observer shell for TabGroupSyncService
>
> This CL adds a shell for a new TabGroupChangeNotifier that will act as
> a delta observer for changes from TabGroupSyncService.
>
> For now it is only a shell, but in later CLs we will add support for
> in fact converting the changes from TabGroupSyncService to what we
> need and matches CollaborationEvent type changes.
>
> Since we with this CL is starting to fill in the
> MessagingBackendServiceImpl, this CL also adds a test suite for this
> class.
>
> Bug: 345856704
> Change-Id: Ie7a3788d05f94baa131d373b9127ef7cbbeb309d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5985139
> Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
> Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1379270}

Bug: 345856704
Change-Id: I0b4b0e7627c41120a56b2f91aa588b1143beabb7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5999715
Owners-Override: Igor Kraskevich <kraskevich@google.com>
Auto-Submit: Igor Kraskevich <kraskevich@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1379584}