Remove AuthenticationService::GetAuthenticatedUserEmail

Most of the call-sites are only interested whether the user is
authenticated (i.e. they check whether the value returned it not
nil) so convert them to instead use the IsAuthenticatedMethod()
method.

The only call-site that used the returned value can instead be
converted to call GetAuthenticatedIdentity() and use -userEmail
property.

Bug: none
Change-Id: Iac07ad957adef4065cafdf8b681c7677dd674cdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574800
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653126}
9 files changed