[FedCM] Remove single IDP members from AccountSelectionMediator

This CL has no UI change. It removes the IDP and client metadata members
from AccountSelectionMediator since having these assumes single IDP UI.
It may be needed to add a list in the future, but for now it suffices to
access the relevant IDP from the Account.

Bug: 392142580
Change-Id: I25f11ed7c6c05b3589dbf45955eb6836c388b291
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6305540
Commit-Queue: Nicolás Peña <npm@chromium.org>
Auto-Submit: Nicolás Peña <npm@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425738}