Convert ConsentLevel::kSync usage in ChromeAutofillClient

This replaces a usage of ConsentLevel::kSync in GetAccountHolderName()
with ConsentLevel::kSignin - there's no reason this particular flow
("card name fix flow") should be tied to Sync consent.
The equivalent change was made on iOS in crrev.com/c/4746809.

GetAccountHolderEmail() and IsMultipleAccountUser() were unused and
are cleaned up.

Bug: 40067770
Change-Id: Ie0f490f3fcb1a0016e28114d0a7f668ae611f14a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5340171
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267885}