[WebID] Remove identity sharing prompt from navigation throttle

The WebID navigation throttle was initially added to demonstrate how
the permission flow could work in a fully backward compatible manner,
with permission prompts associated for tracking risks in both
directions.

It is now being adapted to demonstrate session management capabilities
that are aiming at the deprecation of third-party cookies. The second
prompt, at the time the OAuth response is being returned to the relying
party, is not necessary in that context.

Bug:1223570

Change-Id: Ie4481770eebf5fccbe760181c9327af78047e1a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2986382
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#896129}