[Autofill] Migrate OnCADWillBeDeleted() to OnCADStateChanged()

This CL deletes CADF::Observer::OnCADWillBeDeleted() and replaces
it with CADF::Observer::OnCADStateChanged().

A followup CL will introduce platform-independent
AutofillDriverFactory observers.

Bug: 354043640
Change-Id: Iaa55ea907d7fff6dedab2c973dcc70ad27dc80ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5741464
Commit-Queue: Christoph Schwering <schwering@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1334122}