Remove devicetokenAPI calls from ExisitngUserController

PasswordSyncCheckers have been added to the ExistingUserController to
perform password sync token check for SAML users in case of hidden user
pods. In the normal case (user pods visible) the logic is owned by the
USerSelectionScreen.

At the moment this functionality looks redundant - if user pods are
hidden, users will be sent through the online flow by default and will
fetch updated sync tokens on starting a new session.

Bug: b/269284622
Change-Id: Ic8d7b3677c3dfdf1073d95811962500053d4b9ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4531864


Commit-Queue: Maciek Slusarczyk <mslus@chromium.org>
Reviewed-by: Renato Silva <rrsilva@google.com>
Reviewed-by: Mohammed Abdon <mohammedabdon@chromium.org>
Reviewed-by: Roman Sorokin <rsorokin@google.com>
Cr-Commit-Position: refs/heads/main@{#1143946}