[FedCM] Remove some last_used_timestamp leftovers in desktop UI code

This is no longer used, so it can be removed. Note that in theory we
could also remove it from the content/public interface, but it would
make it a bit messier to perform the sorting which relies on this
timestamp. Because of this, we keep this, at least while we keep using
it for sorting purposes.

Fixed: 404220775
Change-Id: I879edcb1953d70bac0a44d1fb6ac86bd3e25efac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6702574
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1482695}