Fix SendTabToSelfCoordinator potentially waiting device list forever

This touches a user flow behind a feature flag disabled by default.
This UI signs in then waits for sync to download the list of target
devices, via an observer. However, before this CL, the observer was
only removed in case of success, meaning it could stay around forever
if sync never responded with the device list.

With this CL, the observer will be removed if the user dismisses the
UI in the meantime.

Bug: 1219434
Change-Id: I13fcfafeabe1cdeb7950837e29ef108ed6b9638b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3657651
Reviewed-by: Sophey Dong <sophey@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1006534}