[iOS] Make sure EnterpriseIdentityServiceImpl removes observer

This patch tries to understand why IdentityManager still has an
observer on shutdown.

Add:
void EnterpriseIdentityServiceImpl::OnIdentityManagerShutdown(
    signin::IdentityManager* identity_manager) {
  // Needs to be shutdown before IdentityManager.
  NOTREACHED(base::NotFatalUntil::M142);
}

Bug: 410122787
Change-Id: I1423ec5d979f22aa21c43b4cb3b8cea6083312e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6460226
Reviewed-by: Fabio Tirelo <ftirelo@chromium.org>
Commit-Queue: Fabio Tirelo <ftirelo@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448515}