Remove DiagnosticObserver::NotifySigninValueChanged() and its usage

The semantics of this observer method are strange and unclear, and
rather than fixing it up, we can just eliminate it as it is used only
in legacy code.

By removing this we can cleanup the API surface of SigninManager and
IdentityManager.

Bug: 913850
Change-Id: I7e7ebc455eaab208d57b91f7398055f62a915af5
Reviewed-on: https://chromium-review.googlesource.com/c/1373453
Commit-Queue: Sergio Villar <svillar@igalia.com>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616665}
9 files changed