[ios] Remove unused methods of AuthenticationServiceFactory

Remove deprecated methods of AuthenticationServiceFactory now that
all client code have been migrated away from them.

This is part of a multi-CLs change:
  - https://crrev.com/c/5975561
  - https://crrev.com/i/7784946
 => https://crrev.com/c/5972830

Bug: none
Change-Id: I86a0d9f770f9486d85a34dd6bb888875d2606912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5972830
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1376371}