[signin] Cleanup site isolation experimental code

Site isolation for Gaia is now launched at 100%, and we can remove the
codepath that was used when site isolation was disabled.

In particular, after
https://chromium-review.googlesource.com/c/chromium/src/+/1096664
the GaiaOriginIsolatedCallback always returns true, and can be removed.

TBR=blundell

Change-Id: If41007a3fc0b94c58a56f8f58a35f034861ae015
Reviewed-on: https://chromium-review.googlesource.com/1100466
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567216}
9 files changed