Remove the Supervision Onboarding screen and handler.

Bug: 977566
Change-Id: I84ec81f7fd20fe9dd122602a3884efe4f0e3e1e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671408
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Lucas TenĂ³rio <ltenorio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671790}
19 files changed